Because disallowing those link redirection services would hurt the platform's profits by adding friction for advertisers. Link fraud makes them more money and they aren't punished for it.

These platforms operate more like oligarch crime lords than friends of society. There's no "free market" solution to fleecing the public, it's lucrative and all the players participate in it.

If browsers can load content from a different place than what they display someone is going to use that.