Although the out-of-universe answer is 'because God knows this comic didn't need any more characters,' is there any particular reason that everyone dead in the dream bubbles is from Troll/Human related session? (Does God-tier dead Callie count? Did she know the humans in that iteration of her life?) Why didn't anyone ever run-into a dead squiddle player from the 48 squiddles session?

The 48 player Squiddle session is non-canonical. Alt-Callie never met the humans from her timeline. The dreambubbles we see were made specifically for the trolls and their related sessions on request of Feferi.