Because they’re designed for a common denominator audience.

It takes exceptional skill to make a multi-layered script that talks at various levels. It’s rare enough that we’re shocked when we encounter it - perhaps in older kids movies, maybe in things like the Lego movie.

It’s much easier to rewrite various similar stories with various tropes mixed in. Not to say these are not enjoyable! Many times you just want to see the same story different this time.

Think of all the romance novels pumped out that have the same basic plot, but the people who read them demand more and more.