Small businesses don't want to pay for simple apps. They want to pay a percentage of their sales to a large middle man who takes care of sales. It's too much for them to pay a few thousand (or hundred) dollars for a bespoke solution, but they have no problem paying tens of thousands per year to a middle man.
Almost none of those reasons are managing what are generally trivial costs for a growing business.
Small businesses remain small because they: cant scale, have a small tam, require direct labor from the founder to get the job done, the market is fairly well established but the founder has some special skills which are difficult to impart, especially in the context of the business.
Market conditions, the ability to get capital, and a lot of luck have way more to do than is your SaaS bill a little too high, you can always just use some google sheets and google docs and get 80% of what you'd need in most SMB situations.
When you deal with enough small businesses, you'll find out that they usually pay a large percentage of their revenue to middle men in order to save a pittance of money or effort which would be required to get rid of the middle man. People don't act rationally. Those who do act rationally regarding their business, generally find ways to grow it beyond being a small business. Assuming they want to. Many are happy being small.
SaaS is not a middle man. They're usually a good solution for a small business.
Pretty much everything boils down to having capital. Tech startups work because they inject capital into an idea that can be developed using technology. If you don't have capital to hire talent and sustain losses during several years, you have little chance to create a growing business in the modern economy.
There's a reason small businesses remain small.