I run[1] an open-source hosting company focused on privacy, so self-hosting everything[1] we use provides an extra layer of guarantees regarding security and privacy.
Plus, if something goes down, we have no one to blame but ourselves, which I see as a nice guarantee for customers as well — there is no lying or blaming the other guy.
[1] That is being generous. We have a few customers in a closed beta, but need to wait until the European borders are open before launching.
[2] Except for our XMPP, email, and EteSync servers, we self-host everything else. These will eventually be self-hosted as well.