While it's true that there's a huge amount of essential complexity in this kind of software, there's also a lot of completely unnecessary bad UX and user-hostile design. This is caused by lock-in and not selling to the end-user. If they don't think bad UX will lose them any customers, no effort will be put into it.

It can work for a long time, but eventually it does create opportunities for competitors.

You're absolutely right that there's a floor for how simple these products can be, but most of the incumbents aren't anywhere close to that.