Because telecom is dramatically more difficult to solve and the regulators don't have the competence - or the legal standing - to implement a good outcome.
With Facebook you can order them to split off Instagram and WhatsApp. That's a very feasible thing to do technically, and could be accomplished within a year. It's straight-forward compared to dealing with telecom.
So you split Comcast into a dozen baby Comcasts. You just changed absolutely nothing (and plausibly made the situation worse for a lot of people, as some of those baby Comcasts will suck even worse over time) because it's a physical location problem, it's geographically limited competition (unless you redo the entire regulatory structure re telecom). Each baby Comcast will dominate their little territory and then the re-combination will begin again, as it did with AT&T and the baby bells. Regulate that outcome away? That's the part where the competence and legal standing is a huge problem. Make it so the pipes are forced open and lots of competitors can use them? Un huh. You would have to entirely remake the telecom industry in the US, get us entirely different Congress and regulators, and overall that isn't going to happen for a few dozen reasons that would require paragraphs to explain fully. You know it's not going to happen, I know it's not going to happen, everyone knows it's not going to happen. So what would going after telecom accomplish? Not much most likely.
What does it even mean to split WhatsApp/Instagram? That changes nothing. They aren't even now connected. The only useful feature is to crosspost which barely anyone knows how to do and it's super useful for small NGO's and such where bigger players can just pay someone to do it manually.
Messenger and Instagram are now connected [0]. WhatsApp is one step away from being connected and integrated [1]. Even if they are not technically integrated, they will be logically integrated - one way or the other.
