Maybe Facebook has bigger problems because they’re so huge; like being a bigger target for attack by hate groups.

Conversely, the fact that they are one single corporation lets them achieve economies of scale and reduce their operational costs per user.

Maybe they just really like their fancy offices and cafeterias.

Yeah, so what? Do you think that the developers of free software, the admins of the instances and the moderators putting in time to make this work don't deserve recognition/compensation for their work?

You are basically saying that only martyrs should be doing work on FOSS, the Fediverse and anything that is based on a good ethical foundation. It's basically giving the middle finger to the people who can actually make a difference.

better for the world if online speech is diversified over lots of small services

Absolutely agree. The more decentralized, the more resilient we become. However, the cost per user does not go down, in fact it goes up. Running the infrastructure to serve 2 billion people (like Instagram/Facebook/WhatsApp) requires massive resources already in a centralized/highly optimized corporation, on a decentralized structure it will cost even more. The question is: are the people willing to bear these costs? So far, the data says "no, they are not".