The same reason spam filtering is hard. It's not possible to catch every misuse of the service without too many false positives.
It's probably possible to catch a lot more of them, but why look too hard when you can hide behind section 230 immunity and pocket ginormous profits instead of spending on this lol...
Ok, it's even harder when you do not care because they people are either freeloaders or locked into your solution because it's a customized mess.
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The same 5 urls has been used for 3 months