I guess it's just because the difference between BC and BP is irrelevant for most archeological and geological purposes, and referring to a specific year for so

I guess it's just because the difference between BC and BP is irrelevant for most archeological and geological purposes, and referring to a specific year for something that happened 200 million years ago doesn't seem to make much sense, so "present" is a suitably vague indication. But then you have special cases like this one, recent enough that the difference is actually 50%, and you need to specify a more precise time point for present. So, as an afterthought, you decide that "present" is 1950, but you still use a uniform scale for all events.