I suspect this is because they are usually introduced like "here are bitfields, but DO NOT USE THEM for anything that might become ABI because their order is implementation defined". I think that causes people to mentally file it under when actually there are many contexts where the ABI risk is not real.
Meaning, if you have a struct and you want to bit-pack your booleans, you can declare each one as, say, uint8_t some_bool : 1;
You may then do `x.some_bool = true;` etc.
It's a small nicety to avoid bitwise operators, anyway.