The issue is that XSD came along much later, and its use did not become binding in XML validation scenarios, hence partial success, even when the XSD-based validation tooling was available at the time.

XSD provides a clean abstraction for the technical validation that sits separately from the application / business / processing layers and dramatically increases the chances of a «clean» request reaching the aforementioned layers without having to roll multiple defensive checks in there.

Granted, an XSD can become complex very quickly, especially if indulged in too much, but it does not have to be.