Partly this the fault of Apple. They championed the idea that UI is an artistic expression and therefore 'configurability' and 'customizability' is akin to heresy. This view has now infected many existing products.
Part of the reason why many companies jumped on this bandwagon is also because 'customizability' is hard(er) to build in, and certainly more expensive to maintain.
It's also harder to A-B test with so many variables. If tests aren't statistics significant, the value of user analytics and UX experimentation decreases from a "% lift" perspective. It's harder to know if a feature change or a user defined config had a causal relationship to some other metric.
It may be the A-B testing tail wagging the dog.
Those types of tests serve two very different purposes. UI is also unit testable.
Unit tests are more of a binary pass/fail. A-B tests are looking for cause-and-effect relationships by comparing some metric between a control group and a variant group.