Isn't the earth's high suitability for life a good reason to believe in a designer?

Turn it the other way around: Earth is not fit for life, life is fit for Earth. Terrestrial life evolved here, under Earth’s environment, so of course we are fit for it. If Earth’s rotation or revolution or distance from the sun was slightly different but still able to support life, it’s possible that entirely different organisms would be sitting around contemplating profound-sounding inanities like the anthropic principle. This claim means to demonstrate the significance of life but relies on an assumption of the same to make any sense at all. It needs to find its way into the philosophical dustbin.

Our galaxy, the Milky Way, contains hundreds of billions of stars.

And the universe contains about a hundred BILLION galaxies. The other galaxies we know about lead us to assume that hundreds of billions of stars per galaxy is a reasonable guess.

This brings us to a broad estimate of something like a billion TRILLION stars in the universe. The universe is vast beyond our wildest imaginings.

Doesn’t the fact that, so far, so far as we know, life only exists in an infinitessimally small part of the universe, actually point to the very opposite of what you are suggesting? Far from the evidence pointing to a designer who intended to bring about life, it points to life being simply an accidental add-on, something that happens as a result of natural processes when conditions permit, but which most of the conditions in the universe do NOT permit.

If an intelligent designer created the universe with life in mind, it made a pretty poor job of it! Personally, I think the universe is so vast that it is almost impossible to imagine that there aren’t other planets out there capable of supporting some form of life. But that doesn’t alter the fact that the vast vast VAST majority of the universe is, so far as we can tell, completely devoid of the stuff.

There are no mysteries here. There are an almost infinite number of places where life is not. Life can exist only in places where the conditions are able to support it. No surprises, then, that here we are in a place that is able to support us.

Life evolves to fit environments so well it appears to be custom designed.

If the earth were just a bit warmer or colder or spun a bit faster or slower or had a bigger or smaller moon (or no moon at all) life might still have evolved, but into critters that were very different from us. Some of those critters would be saying, “This planet is so perfectly suited to us we must have been designed by gods!”

Of course, they would be very different gods than the ones humans have invented.

Some excellent answers below, but I should also point you to Ebonmuse’s rebuttal of The Case For a Creator and this post about planets in particular.

Also, it’s hard to imagine what other forms of life might arise in different conditions. Consider for example, the Pompeii Worm.

Compare these two questions:

1. If you buy a lottery ticket, how likely are you to win?
2. Among all the people who buy tickets, how likely is it that someone will win?

Now compare these two questions:

1. How likely is it that any particular planet in the universe will be suitable for producing humans?
2. How likely is it that there exists at least one planet in the universe suitable for producing creatures intelligent enough to ask such questions?

The universe is a big place, and there are lots of planets in it.

Among the teleological arguments for the existence of God, there are a couple referred to as the fine tuning arguments. The question focuses on the planetary version of the fine tuning argument. The other (stronger) version has a cosmic intelligence carefully setting the universal constants to allow life to appear. Neither version is compelling.

Dougles Adams’ idea of the sentient puddle is relevant to your question:

[...] imagine a puddle waking up one morning and thinking, ‘this is an interesting world i find myself in – an interesting hole i find myself in – fits me rather neatly, doesn’t it? in fact it fits me staggeringly well, must have been made to have me in it!’

We know that the puddle is mistaken. And I think you’re making the same mistake: Life emerges only where it’s able to (where the conditions are just right like on parts of earth), things are not set out in advance in preparation for life.

It seems to many religious believers that life, and human life in particular, is in need of an explanation. I’m not convinced that it is. I’ll try to explain why.

The odds against any particular bird crapping on your head are spectacular. Is this a good reason to suppose that a cosmic intelligence must have been pulling the strings whenever this kind of messy coincidence occurs? Of course not.

So the mere improbability of an event isn’t enough to convince people that intentionality must have be involved. There needs to be something more.

The extra ingredient is that the outcome in question has to be special.

What exactly special means in this context can be difficult to pin down. But some examples can help.

If I roll a 1000-sided die and the number 852 comes up, we don’t consider this value to be special. But if prior to rolling, I had announced that the die would show 852, the prediction would have made that value special. The agreeing result would not only be improbable (like the pigeon poo), but it would now also seem to be in need of an explanation; was the die weighted? etc.

The question is then: what makes life not only improbable, but also special? It might seem self-evident to you that life is special. But you’re a living being yourself, of course you’re intensely interested in life. This is a bias none of us can escape.

From the perspective of a living being (and as far as we know there are no other perspectives), life seems special. But is life special in some absolute sense? We have no reason to suppose it is, and no way of knowing.

Of course we’re interested in how life got started, but the claim that the emergence of life is a phenomenon in special need of an explanation, and one that demands a cosmic intentionality, is unfounded.