^The point is, partly, that they don't assume the same people are reading comics decade after decade. The reboots are usually done in order to make the continuity more accessible to new readers, to make a fresh start as a jumping-on point for an audience that might otherwise be scared off by all the complicated backstory. Comics have a lot of trouble attracting new readers these days, so they'll do whatever they think can help them do so, even at the risk of alienating the old guard.
In particular, the New 52 reboot was done for the sake of the emerging digital comics market, to allow digital-only readers who were trying the comics for the first time to come in at the beginning of the story rather than having to come in at the middle and play catch-up. Although exceptions were made for the already highly popular stuff like Grant Morrison's Batman and Geoff Johns's Green Lantern, which were allowed to keep their continuities and story arcs largely intact.
Although usually what happens is that new readers don't come onboard to the extent they were hoping, while the established fanbase continues to read the comics, and new creators and editors come in who were fans of comics growing up and want to read stories like the classics, so they reintroduce the old ideas that previous creators and editors got rid of.