Fair point, but there’s a reason. It’s a browser game that runs on its own server and keeps your hero in your account, so it can’t run inside itch’s frame. That’s why the button opens it in a new tab.
Honestly, I think that’s still better than downloading and installing something. You can play on your phone, a tablet, whatever you’ve got.
It’s just a fan project. I made it for myself and later decided to share it. If it’s not for you, that’s fine, nobody’s making you play. But if you’ve played PoE, give it a try. I put everything I love about PoE into this game, so as a fan you’ll probably enjoy it.