How come the original Skyrim has a PEGI rating in the UK when the BBFC was still under use at the time?

Skyrim was released in 2011, but PEGI didn't take over from BBFC till 2012

People still pay attention to ratings?

Perhaps they didn't update it. Maybe an oversight.

Some people do because they know nothing about the game and may be gifting it. Also there are sometimes legal reasons to make sure it's ok.