Hundreds of thousands of people are on the fediverse, and many large projects use email every day for their work - Linux, GNU, Postgres, Debian, etc. It might not be for you, but it works for many people.
I haven't been on Mastodon in a while, but when I was, it was far from a ghost town. You see what you federate with, and following or engaging with more people will fill your timeline up more. If you don't use it much, then you won't get much out of it.
No, it's nothing like Twitter in terms of DAU. However, your Twitter account compared to your Fediverse account is not a good representation of the entire state of the Fediverse. And: it's not necessary to capture 100% of the market share on attention. It's simply necessary to capture enough that your project is successful.
It’s the only social media I use, and it’s a ghost town. We have to live in the world that is, not the one we wish were.
I used to have a Twitter follower count on my niche little tech/privacy account that is higher than the DAU count of the entire fediverse.