Good point! For people who have tons of experience with serverless, serverless is probably a better choice than running servers for some use cases.
As a small business owner, there are two types of cost that I need to consider:
Time: the time I use to do A is the time I can't use to do B. Unfortunately I haven't used serverless so far in my professional career -- in this sense, I'm not full-stack enough :) It takes time for me to learn it, understand it, operate it, and experience various outage scenarios to gain the true learnings. It's more costly for me (probably not for others) to use serverless than the things that I already understand. I'd rather spend more time on other non-engineering things nowadays -- believe it or not, I spend 1/3 of my working hours replying emails :)
Money: the money I spend on A is the money I can't spend on B. I decided not to use api-gateway + lambda & hosted db servers, primarily because of $$$. I actually did the pricing calculation a few times last year. In addition, api-gateway + lambda also require some time for me to learn, which I should use to talk to users, marketing, building new product feature, thinking (yep, thinking also uses some time budget :)...