I think one of the benefits of the MCU is death can generally be permanent. There are exceptions of course. Ultron can easily have slipped a kernel of his AI somewhere and come back. Somebody like Red Skull was never confirmed dead, and in fact for years what happened (got teleported away) was theorized about. Everybody that got dusted can and likely will come back.
But bringing back other people with definitive deaths not tied to events that are "easily" reversed or explained will weaken the MCU. Even logically actors age and grow tired of playing characters or never wanted to be in a bunch of movies. It's a necessity to eventually kill off or retire characters, so why not avoid meddling with unnecessarily bringing back dead characters?
EDIT: I would also add that occasionally you can play with the idea of bringing back a dead character and it going wrong, but that's a very precarious storytelling device. The idea of Wanda gaining or realizing she has more power and bringing back her brother only for him to come back without a soul and evil could work, but given the relatively short period we have for something like that, again, it's probably best to be avoided.
