Because eventually bad actors take any decentralized platform / standard and ruin it for the rest of us, leading us to trust the few good players that remain (see: email). Sure, technically you can spin up your own mail server -- but because of the copious amount of spam from people who have done that in the past, you'll go through so many hoops that eventually you'll throw in the towel and probably use GSuite or a known major provider.

As Ben Thompson says:

> [...] centralization is a second order effect of decentralization: when all constraints on content are removed, more power than ever accrues to the entity that is the preferred choice for navigating that content.