How can atheists refute that God created Earth, being a perfect planet for the existence of Humanity?

Let me introduce you to a certain puddle of water introduced to us by Douglas Adams:

There is this puddle of water that looked at its surroundings and thought to itself that things seem to favour it’s existence. The puddle argues that the hole fits it really well. It argues that without such a nice hole, it wouldn’t be able to exist. As such, it goes on to conclude that the hole must have been there for it. No matter how many times all the problems with such reasoning is pointed out to the puddle, it insists on how perfect the hole in the ground is. I hope you aren’t that kind. As such, I’ll try to share some thoughts in the hope that you’ll see why your hole, the earth, isn’t really perfect.

Let’s start with a map of the Earth. Why? Well! We will see:

About 70 percent of this planet is covered in water and most of that is the kind you can’t even drink! So, if I were to randomly drop you on a random spot on that map, you are more than twice as likely to land in an inhospitable environment where you’d just die. What about that gives you the idea that it’s perfect for you?

Then we have the problem of food production. This planet doesn’t have enough built-in resources to support your kind (the 7 billion people). Not only are there places where water is so scarce that growing food is next to impossible, there are a lot of places with climate so harsh and cold that it kills the land:

So it’s not like of the remaining 30 percent of the Earth you get to use, you really get to use it all. So what about the bits that we do get to use is so perfect? Let’s see what is built into our environment that makes it so perfect for many:

The predators! Without the kind of man-made artificial civilisations and the safety they can provide, this planet leaves you at the mercy of ruthless predators that will see you as nothing but lunch. They won’t think twice before they go after the weakest ones among us. What a perfect environment for you and I! Just when you think we’ve had enough of this perfection, you realise that there’s always more.

Just when you think the world can’t be any more perfect, you get reminded about all the dangerous diseases, millions of them, that are constantly trying to harm you. Without the level of sanitation, vaccination and medication we have developed, many of these invisible carriers of doom could have single-handedly wiped out most of us. What kind of a perfect world wouldn’t have such deadly pathogens just roaming around all around us, right?

I think it’s about time I summarise the essence of the point I’m trying to make here. You see, we tend to live on tiny hospitable segments of this planet. Over thousands of years (probably more), we have found and colonised the tiny bits of this planet that won’t kill us as quickly as most places on earth would. We have invested incalculable amount of resources over many generations to create the artificial safety and habitability within the confines of wherever we set up camp. After the thousands of years of engineering to shelter you, after the thousands of years of medical science to protect you, after the thousands of years of agricultural science to sustain you, you get to finally have some peace and feel very safe and cared for in your tiny pocket of residence on this planet.

Don’t get me wrong. I am happy for you! I am truly happy that in the narrowly confined segment of this planet where you live, it appears perfect to you. That’s a good thing. However, what I don’t really appreciate is you then erroneously projecting that sense of safety and fulfilment onto the rest of the planet. For the kind of reasons I’ve pointed out in this answer, it should be clear to you now why no matter how good your local conditions are on this planet, you aren’t justified in extrapolating that condition and making such extraordinary claims about the global condition. Having said all that, I’ll try address the remaining bit of your question:

How do I explain living on such a perfect planet?

I am not convinced that this planet is as perfect as you make it seem to be.
That leaves us with the question of how I would explain existing on this planet. My answer to that question is similar to the answer I’d have given to the puddle at the start of this answer:

One could only exist in a place where it’s possible for them to exist. The miracle, or interestingly noticeable case, would be in one existing in a place where it’s impossible for them to exist.
It’s for very similar reasoning and sentiments that I’ve never understood why a God would ever get a teenage girl pregnant to show me the power of miracles. After all, finding pregnant teenage girls is not outside the realm of possibilities. If God really wanted to show of his miracle, getting a teenage boy pregnant(impossible) would work as a much better hint for a potentially divine intervention.