In their defense, we're talking about characters that have been around for up to 70+ years, most of whom had at least one, and sometimes several, comic books about them come out every month for many of those decades, and since the rise of the direct market in the last 30 years or so, have mostly been bought by the same hardcore group of 16-40 year old neckbeards who thrive on the continuity--and the vast majority of writers, artists, and editors come from the same group of fans. So what do you do? Appease the few thousand hardcores who are keeping you afloat, or try to branch out and risk failing utterly? Once in a while, Marvel or DC try to do both, which is where things like Crisis on Infinite Earths, the Marvel Ultimates line, and the New 52 come from. But generally, they stay the course and appeal to the existing fanbase, which isn't smart long-term thinking, but there you go.
The more successful movie and series adaptations of superhero comics tend to be those that jettison most of the continuity that only appeals to those few thousand hardcore fans, while focusing on the foundations of the characters, which have more universal appeal. E.g. teenaged nerd gets spider-like superpowers, fails to stop a criminal from killing his uncle, and now feels responsible for fighting crime. Really easy to understand. That same guy married to a supermodel, but in actuality a clone of himself, and he and his wife sell their wedded bliss to the Devil to bring his aunt back to life? WTF?
