If most people use a single service anyway, why would a company go out of its way to make something decentralized?
Same reason they did it for email?
What, by starting as a government system using a completely different protocol, then adapting to always-online network connections (i.e. universities) at a time when spam didn’t really exist?
The 70s and 80s were a very different time, and regular consumers didn’t use email until it had gone through several iterations. Even so, most people used a single implementation (sendmail on BSD) for quite some time before anyone else got involved.
The internet today is a very different beast, you can either try for an open standard, or you can try for user acquisition. Almost nobody seriously goes for the open standard anymore, unless it’s an iteration of an already existing open standard.
