Because some people will only ever be happy in hell. And if you’re happy in hell you’re gonna be there for a while, probably a stretch of time you can only think of as ‘forever’.
A few things you need to keep in mind when it comes to Biblical theology. First, forever / eternity means as long as we can remember. It’s not to be taken literally. God promised David his kingdom would last forever, when in fact it only could have ever lasted as long as the Israelites could remember their covenant with their Lord. When the Israelites forgot God, their covenant ended and God send His chosen people to live as slaves to the Babylonians.
Second is that God didn’t make Hell, we make Hell through our own wishes and wants and desires. Any time we turn our minds away from love, truth, and goodness, we are existing in Hell. It’s not a problem, humans move from Heaven to Hell to the realm in between pretty much constantly. It’s when you come to love focus on the self as opposed to focus on the Lord that you put yourself on the road to Hell. And once you fall in love with one of these fundamental spiritual concepts, it’s near-impossible to change your mind ethically.
So God provides a safe place for people who love self to live their preferred lives. The people that live there don’t feel like they’re suffering. This doesn’t mean they’re not actually suffering, anybody who studies Buddhism knows that just existing implies suffering. You don’t have to feel like you’re suffering in order to suffer. That’s what hell is like.
Anybody who wants to go to Heaven can go. There aren’t any locks or bars. You just decide you’ve had enough of this kind of life and wander on over where everybody’s warm and peaceful. It’s just that if you’ve come to love Hell in your Earthly life, you won’t feel warm and peaceful near Heaven, you’ll feel put upon and attacked. Like bringing a Bible to an atheist convention.
God’s not demanding or promising, he’s making a prediction based on His infinite wisdom and understanding. He wants us to choose Heaven over Hell. And He knows that not everyone will love Heaven. These people will find fulfillment in Hell. And because that’s where their fulfillment is, they’ll be there for a long time, long enough to qualify as forever.