Because it is deliberate. The whole appeal of posting porn on Reddit, is that unlike proper porn sites like onlyfans where you have to provide proper age verification such as an ID or drivers licence, which will then be linked to your real name, all you need to do is convince some Reddit mod that you’re over 18 on a “verification post”. It’s a much lower barrier to entry, which is how Reddit gets people to post on there. The Reddit algorithm is also much easier to manipulate than other sites, so creators that want to make money can just spam their posts everywhere that link to a PayPal or paid onlyfans account in their bio. Extreme fetish content not allowed on mainstream sites is also just allowed to exist on Reddit. Such a system is ripe for abuse of course, but Reddit doesn’t care because of the huge amount of traffic it generates for their website. They just try sweep it under the rug everytime. Look at what happened to Tumblr when they were forced to deal with the porn issue, the site was never the same afterwards.