Brand new to POE, why is there EXP loss post 80 & EXP gating?

PoE is the worst game I know, in how designers treat their players effort and investment. And not because they have made a difficult game, but because they are lying by design.

For example Elden Ring is also difficult and hardcore, yet you can beat any boss (or ambush encounter, which may be even more dangerous) while being drastically underleveled and undergeared.

In PoE, they greet you with plentiful variety, and only much later you discover that either you build exactly as they want (and they want to constantly twist balance and force you to redo theorycrafting and grinding from scratch), or you won't progress an inch further, regardless of your personal skills or preferences.