[pphysch]

This differentiation is amplified in (pure) mathematics, where two different subfields can have essentially zero overlap*, making peer review and general QA difficult to scale.

* Physical sciences also have a lot of diversity, but at least you can go to their labs and see their equipment, reagents, data, etc cetera.

[blauditore]

Yes, this is essentially because mathematics is such an old science and goes very deep into branches. That's btw also why getting a certain degree is much harder there than in any other fields I've seen (did a bunch of courses at university in various fields, and the diff between e.g. pure math and psychology is almost comical).