ShoC was very much HL2 with rpg elements - just run through maps and have fun dealing with shit the game throws at you.
Then CS and CoP introduced this more "complete" gameplay loop of circling the same areas doing quests for faction progress and loot.
Modders can't do shadow of chernobyl because they'd need new maps and setpieces, and we've been reusing the same concepts from 20 year old design documents.
There is no surprise or thrill here anymore, it's banal stagnation.