Why do movie sequels tend to get worse in quality and video game sequels tend to get better in quality?
That's debatable. It's all a matter of personal opinion.
Go to any forum site for video game sequels, and you will find people saying the later games are worse ... Or lack something ... Or ways to improve them.
Of course, if you're talking strictly about graphics for video games ... even if gameplay is worse, graphics will continue to get betting in quality simply due to advancing technology.
That is not always the case, many would say that The Godfather 2 was better than the first one. Red 2 was better than Red.