This is a side effect of how governments, particularly the US but also Australia, organize people. In the US, "White/Caucasian" is a group you self-identify as on forms and stuff. They are paired together from childhood, even if the reality is "people from spain are caucasian". In the US, "Caucasian" literally means "White American" and there is no further thought/purpose to the term. It is not expressly the same as the outdated racial classification.
Likewise, a lot of institutionalized racism leads people to conclusions like "spanish people and everyone in the middle east are two sets of brown people" which...is...not accurate, and both are also considered caucasian regardless.
It's similar to when people in America use the term "African American" to mean black. It's just general day-to-day ignorance of what you're actually saying, and usually comes from a place of trying to be accurate, as we had a period where it was considered questionable to simply refer to someone as black and everyone over-compensated with "African American", and a bunch of people started using it to refer to literally anyone black- even if they weren't american! By mistake or ignorance, it was a thing. Nowadays we're back on simply using the term Black.
It's a very complicated thing, which is part of why the conversation on Sims 4 skintones is so hard to have- because not everyone has all this cultural context, and to some people, "But what about the lighter skins?" reads as "But what about the white people?". Unfortunately the OP reads a bit like the latter, but the former is a valid issue to address. The light skins in this game are a quality issue now, same as the dark skins were before.