> Why Isn't the Construction of CRUD Web UI's 90% Automated?

It is, several times over.

Using SQL or some other domain-specific query language against structured records? That's the first 90%. HTTP and hypertext? That's the next 90%. CSS and modern HTML authoring? Another 90%. Web app framework with an ORM and MVC / data binding / reactive / flavor of the month? Another 90%.

Your mistake is assuming that automating 90% of it reduces the work by 90%, rather than letting you spend 10x as much time on making the bespoke parts fancier.