Because it is, for many founders. Those who think, "I want to start a startup, make lots of money. A SaaS business is the easiest to start and most investor-friendly." What the business will actually do is irrelevant, something to be picked along the way.
I see it as a vertical too - a column of a larger vertical I call "toilet paper companies" (with no offense meant to actual toilet paper manufacturers and sellers): companies run by people who couldn't care less about what the company is actually producing/offering, and have zero interest in the thing they're selling (beyond pretending to, for marketing reasons). Companies that would gladly switch from making rocket parts to making toilet paper, if they believed it has better long-term ROI.
And yes, people who think "I want to start a one time payment ecommerce business" (aka. "a web store") are a part of that larger vertical too.