Why would they voluntarily act as agents of the state to do that? They would need to be compelled legally to do so, and to do that in the US would require a legal order that has not yet succeeded (remember the Apple case around the terrorist in San Bernadino?).

Before you say, "Well, it could be done in secret", please look up the Wiretap + technical assist laws and note the disclosure requirements.

Because if you don't trust them, it's plausible that some or all of the people running the infrastructure aren't actually privacy activists, but moles planted by the state, acting as agents of the state because they are employed by the state, paid by the state, and specifically tasked to pretend to be activists and build a trap.

(There may be some solid evidence making this unlikely in reality, but that ultimately relies on some form of trust.)