Because with great power comes great responsibility, and companies like to cherry-pick only one out of two. It's in the interest of society that when something becomes mainstream enough to be considered "infrastructure" - something with so little competition that ignoring or bypassing it is infeasible given the state of the market - that this infrastructure works within frameworks that we've built and refined over centuries.
Rent-seeking is bad, it's facilitated by ownership of too large a share of the market. In return for allowing Facebook to leverage its size and influence for rent-seeking behavior, we as a society, a.k.a. the government representing us, have the right and responsibility to set the ground rules for what we get in return.
That's not about fault or entitlement, it's about how we can continue to uphold the values that are important to us. Free speech and the ability to publicly disagree are a crucial part of that. If your platform gets popular enough that strengthening or suppressing your voice has a material impact, profit and shareholder interests can't be the only thing that decides how things are supposed to work. There's a lot more at stake here.