The email analogy breaks down when you consider that most email servers are run by big tech and cost a lot of money to upkeep
Or you can run your own email server for yourself and a few of your family.
There's almost nothing in between gmail and some random person's self hosted email server.
In terms of the fediverse,who the heck is willing to host a lemmy server for 1 million complete strangers? Not many people i think