But not their expenses? Revenue != Profit.

Most of these businesses are software as a service so it is mostly profit. There will be some hosting and marketing costs sure.

Unlike startups they are solo operated and usually won't hire people unless they can really afford it an need it.

Advertising can be very expensive.

90%+ of those IH businesses must be making a profit. They are bootstrapped by frugal people. They aren't likely to blow all their profit on ads once they hit 1M arr.