I think in a lot of cases, e-commerce software is written by developers who have never worked in e-commerce, so don't understand the needs of their customers and have to keep adding bells and whistles to justify their existence. In the process, it makes the software glitchy and unreliable.

100%, I can promise you this is absolutely true. There may be a few products designed by folks who've done e-commerce, but by the time it's not a startup company, the folks who've done the work are offloading their (often 5-10 year out of date) ideas to a team of devs that definitely haven't done anything but develop code.