Of the rest, more than half host their own MX. While that's totally reasonable, it means the percentage of people willing to pay just $3/month to be a customer is even smaller than it seems. And $40/year is the cheapest that one could ever hope "being a customer" would cost.
So, lots of people say they want to be customers, but even when doing so is close to free, very few actually do.
(Nothing wrong with using Google because it's free, only while concurrently claiming to want to be a customer. I agree with your point and I'm a happy FastMail customer. I'm amazed FastMail can make a profit at $40 and it's a huge credit to them that they can.)