Nobody gets fired for choosing SAP, Salesforce etc.

Spending tons of money to get a janky, unreliable system of record, or finding out too late it is missing crucial auditing capabilities, or that it has Big Money bugs, on the other hand, is far worse, especially if you have investors asking what the hell you were thinking.

Your point about users not knowing what they wanted until after the fact is also painfully true. The hardest part about these systems is the people most likely to buy are the ones who have been doing it with a lot of human processes for years. Buying a SaaS or other third party product means having leverage to force them to change to more standard practices. Building in-house means that everyone will fight to high hell to make sure that their special snowflake way of doing things is accounted for and you end up in a worse spot as a result.

People do do it, but unless you work for a company you won't hear about their internal tools or products since they aren't selling them.

Complacency, bad management, revenue growing faster than costs for a decade hiding the problem, politics.

There's multiple people highly involved into maintaining the status quo which do everything to take any responsibility out of them.