Because phone companies and ISPs only facilitate private peer–to–peer connections between a small number of parties, whereas the platforms we’re discussing host often-public interactions involving millions of people.

It’s a fundamentally different problem. If people use my phone company to have racist companies, I don’t have to know or care. Whereas if I had to deal with people saying racist shit every time I went on Twitter, I’d stop using it.

Consider also that most people’s biggest complaint with the phone system is a consequence of them not moderating, i.e. spam calls.