It's because for the most part, there's not really any automated moderation on the site, and the folks running it don't really care too much about copyright or IP laws. Add this to a really generous video upload limit for paid accounts, and people are just sticking whole movies and playthroughs on the platform.
Regardless, the DMCA is made so the copyright holder polices' their own copyright.
Just because X chooses to follow what Twitter faught for, which is following the law, not some extra agreements of upload filters, which ironically stifle potential fair use, like youtube, or causes un required licencing agreements due to litigation threat, parent seems to confelate that as in injustice, when in fact its backwards, they dont actually know if its the right's holder who uploaded it first untill they get a take down notice.