Most people don’t because they dont have the technical skills to maintain a server or because they dont feel it’s worth the effort with open servers existing. A small vps can be rented for less than $5 per month or a mini pc can be bought for $50. Not everyone can afford that, but those prices are not prohibitive for everyone outside the first world.

I do. It can be pretty cheap if you don’t open it up for other users. Otherwise it can get pricy.

You can always run your own instance and federate with everyone

Usually signal for communication, totp for 2fa. I’ve as of yet seen only one site that sends 2fa codes through signal.

I’m really happy that my doctor’s website uses Signal to send the authentication code.

Wouldn’t count on it being up to date. For my country 4 out of five biggest banks in the country are missing.

TOTP or Signal, depending on the use-case