I like these questions!
I am not here to sell folks on Cosmere, so this is just me liking that question and trying to
answer it for myself ...
A. For me...
D&D is a miserable system. It has terrible combat that is sooo tedious. It has no social rules, and there is zero support for character plot development. It's just Diablo with pen and paper. Go, quest, kill, rinse, repeat.
B.
I am sick of generic fantasy, especially ones that lean into a foundation of ripping off Tolkien with Elves, Dwarves, Halflings, and humans, wrapped in a western European mythos patina.
C. I have been roleplaying for over 30 years, and I have made a LOT of custom worlds, setting, monsters, etc. I can do this and its fun. But
sometimes its fun to dig into someone else's ideas and build from there. Hence why i am ok with Warhammer fantasy (but i like 40k better), and Dune, and Star Trek, and Infinity, and L5R, and Vampire the Masquerade.
D. The
forgotten realms and dragonlance settings are bore. I read, played and dug into them decades ago, and they never got better, they just got more nonsense and messy.
Especially Realms, its just nothing everything hodgepodge meh. Mork Borg was far more inspiring.
E.
Numenera and Invisible Sun didn't deliver on the world and exploration intrigue they said they would. good games, fine rules, but overall just not really anything interesting going on to come back to and dig deeper.
F. For all the reasons folks are fleeing to Daggerheart, I left D&D and more. But Daggerheart is a bad system. And so far nobody has earnestly tried to, or been able to convince me otherwise. It is made of all the right ideas, in all the wrong ways. So when I went looking for other high fantasy games, other than the discontinued Conan 2d20 that i rather loved....
there isn't anything else to dig deep into.
......
Also here is some official Q/A that speaks to most of your questions = Interview with Cosmere (link)