The problem is that there are a lot of APIs (even system and system-ish ones) that don't want to specify the order of their fields (or outright differ between platforms). Or that can't use a meaningful order due to ABI compatibility, yet the caller wants to pass fields in a meaningful order.

platform APIs like this are likely much less less than 1% of the things I call in my codebases. The few files i have open right now have absolutely no such call.