Because it is. That doesn't mean it is a bad game, it just means it is overrated. People pretend it is the best thing since sliced bread which it clearly isn't. It is not a perfect game but yet people claim it is. The best things the witcher 3 has going for itself is the great story and quest design while the gameplay is hardly groundbreaking and has flaws.
it is mostly insanely good because of the very involving story
and yes the gameplay and progression and also the skill aspect isn't very exciting
but just for graphics and story alone its insane can't expect anything more
oh you like the lamest combat system in history of rpg with the lamest skillset and crafting system
and a storyline that is nonexistent
the only thing it has going for itself is that it has a big world
that on the other hand feels numb and empty