Because if evil didn't exist there would be no point in heaven and hell? If this life is shitty but temporary and if you're good the one after is perfect why would this life be perfect?
That doesn't address why this god allows hell to exist in the first place though. And it's basically in other words just admitting that god allows evil in the world.
Bertrand Russell also had a response to this argument:
“If you looked at the matter from a scientific point of view, you would say, ‘After all, I only know this world. I do not know about the rest of the universe, but so far as one can argue at all on probabilities one would say that probably this world is a fair sample, and if there is injustice here then the odds are that there is injustice elsewhere also.’ Supposing you got a crate of oranges that you opened, and you found all the top layer of oranges bad, you would not argue: ‘The underneath ones must be good, so as to redress the balance.’ You would say: ‘Probably the whole lot is a bad consignment;’ and that is really what a scientific person would argue about the universe. He would say: ‘Here we find in this world a great deal of injustice, and so far as that goes that is a reason for supposing that justice does not rule in the world; and therefore so far as it goes it affords a moral argument against deity and not in favor of one.’”
-Bertrand Russell, Why I am not a Christian
I've argued this point before (in one of your threads too, Noble). God has a plan in the end. How we get there is another matter. There is a series of paths anyone can take, correct or incorrect, but we all reach it in the end with death. Where we go when we die is determined by how we live. If we were wholesome, generally good people, then we probably go to Heaven. If we were murderers, theives, generally bad people, then Hell it is.
God knows about all of our paths. He merely put us on Earth to see how we act and if we were worthy of Heaven.
I remember talking about this to someone a while ago and he then said that we were all previously in Heaven, but when Satan began his takeover, God placed us on Earth to see whether or not we were on His side as humans.
I'm not sure if I agree with this theory, but I felt that it was worth bringing up.
Again this doesn't address why god allows a place as horrible as Hell to exist given that he's all powerful. On top of that, it doesn't explain evils of the natural world, like earthquakes and hurricanes. It's also saying that god allows evil (like murder), which is going to "test" a person's actions at an innocent person's expense (their life in the case of murder).