It's an incentives problem. At research universities, promotion is contingent on research output, and teaching is often seen as a distraction. At so-called teaching universities, promotion (or even survival) is mainly contingent on throughput, and not measures of pedagogical outcome.
If you are a teaching faculty at a university, it is against your own interests to invest time to develop novel teaching materials. The exception might be writing textbooks, which can be monetized, but typically are a net-negative endeavor.
Unfortunately, this is a problem throughout our various economies, at this point.
Professionalization of management with its easy over-reliance on the simplification of the quantitative - of "metrics" - along with the scales (size) this allows and manner in which fundamental issues get obscured tends to produce these types of results. This is, of course, well known in business schools and efforts are generally made to ensure graduates are aware of some of the downsides of "the quantitative." Unsurprisingly, over time, there is a kind of "forcing" that tends to drive these systems towards the results like you describe.
It's usually the case that imposition of metrics, optimization, etc. - "mathematical methods" - is quite beneficial at first, but once systems are improved in sensible ways based on insights gained through this, less desirable behavior begins to occur. Multiple factors including basic human psychology factor into this ... which I think is getting beyond the scope of what's reasonable to include in this comment.
If you are a teaching faculty at a university, it is against your own interests to invest time to develop novel teaching materials. The exception might be writing textbooks, which can be monetized, but typically are a net-negative endeavor.