A key reason why students write about their extra-curriculars, rather than super-curriculars (subject related stuff) is because many have not engaged in anything subject related which is outside of their curriculum content. A key part of the reason for this, is that they are not genuinely interested in their subject so don’t just automatically do it.
Many of the students applying with great PS also aren’t actually that interested either, but they have been informed that this stuff helps thr PS so have done it.
I think schools and colleges do tell students…. But many just don’t hear the message or think the time until writing the pS is far longer than they imagine. It creeps up on them….and they’ve not done anything related to the course they’re interested in.
In the end, the PS does reveal that most students have little to say further about the subjects they want to study. However, most courses are recruiting - ie will offer to everyone who meets the standard offer requirement, and actually will then take many who miss the offer too. They need bums in seats. Few courses are actually selecting courses - where they have more applicants than they can offer to, given the stats about numbers who will firm/insure etc. it is these that use the PS more.