A big part of it is the storytelling in general - when a movie uses stuff that's already been established elsewhere, time doesn't need to be wasted introducing and establishing every character so you can jump right into the stuff everyone really wants too see. When the vast majority of new movies in a series continue to be good, it drives that even faster.
The MCU in particular has built up so much momentum over the past decade that at this point they could probably put out two or three truly awful movies in a row before it would start to slow them down. They won't need Captain Marvel to drag the franchise out of a pit of mediocrity like Wonder Woman had to do for the DCEU.
Another thing to consider is the fact that pretty much any successful movie these days ends up becoming
a franchise later, or at least gets a sequel or two, so even the good standalone movies rarely remain standalone.
Even movies like Pacific Rim and Frozen which don't *need* sequels are getting them within the next few years.
Thinking back to movies I've enjoyed the most over the years, the most recent one I can think of that wasn't part of a franchise and never got any sequels or prequels was Gladiator (2000)
. Honorable mention goes to Serenity (2005)
, but that doesn't really count because it built directly from the TV series.