Legality and stubbornness.

Same reason we have dozens of languages with very minor differences (not different paradigms), endless discussions on formatting despite opinionated linters/formatters existing, bike shedding meetings, and more.

I think we have a standard for the most part. Most of the work being done today is around expressing business requirements in code. The technical side of the tasks is automatically handled by web frameworks, database engines etc. behind the scenes.

The standard exists it is called wordpress and powers most sites.

To me, this sounds a bit like asking why we still have word processors and printers when photocopiers already exist.

Because what gets built is the knowledge of the system. The CRUD part is an implementation detail.

There are multiple reasons. One of them that doesn't get mentioned much is the incentives of managers. If your team reinvents the wheel, it makes you a "technological leader". Customizing an existing standard would mean that the company does not actually need a team of developers, one of them would be enough, which implies that the company does not need you.

People keep talking about how the programmers often choose technologies to advance their own CVs, as opposed to doing what is the best for the company. Guess what, managers do analogical things all the time. It is the most reliable way to climb up the corporate ladder.