Look at their biggest expenses: Real Estate, Inventory, Staffing. For a little one-man SaaS, those are all zero. And their margins: 35%, as opposed to the ~99% margins for selling software.
The only expense we have in the software world is the cost of building the product in the first place and the time away from other paying work spent doing support and maintenance. (And at most a few hundred bucks a month for hosting). Considering the you can realistically launch a working MVP with 3 weeks work behind it, and iterate out new features while in "maintenance" mode, it's really amazing how cheap and easy it is to get a profitable business going in our world.
I mean, sure, you can certainly build a software business with high expenses and low margins (take funding, staff up big, spend a year building your thing before shipping, acquire tens of millions of users with no revenue model). But the "mom & pop" model for software just blows the doors off the mom & pop model for retail.