They can, but up until Firefox legitimized this practice, they didn't, maybe except some malware.

> DoH/DoT is showing up in operating system resolvers just not as fast as apps like browsers were willing/able to add it.

The browsers were so fast, that they skipped the discussion about ramification of this change with the rest of community and just abused their position. One might even wonder, why.

Does not make for good relations in future.

> Standard DHCP options for defining DoH details are still missing though

Yup. Here, browsers are not using their position to finish their push, so maybe the situation is acceptable for them.