Can anyone explain to someone who doesn’t get it? How can a platformer be this great? I’m obviously not a player who gravitates to platformers, but I’m just curious about all this attention.

Platformers can be great (e. g. Celeste), but I don’t think it’s how you describe Hollow Knight genre. It’s a metroidvania in the first place. But anyway, it’s not a genre that brings the greatness, but amazing attention to detail; extremely satisfying gameplay; peak artstyle, music, writing and sense of humour; and amazing plot-twists. As you progress through the game, it reveals you the aspects you didn’t even think about, and it just blows your mind. By the end of your playthrough you feel like you’re playing absolutely different game than you started. I don’t want to go much into details because of spoilers. That’s what would be best to experience yourself if you decide to give the game a shot.