firstly, because that is what the legal notice printed on the release would actually state: eg…
“This compilation ℗ 2016 Sony Music Entertainment …” — it specifically tells you that the phonographic copyright year refers to the compilation. often, though not always, such compilations might further detail the individual ℗ dates of the tracks (which invariably would be earlier than that of the compilation ℗ year), so those would apply to the associated recordings, and would be true for all other releases those recordings are also included in.
if we added all compilation ℗ years to the recordings, and we have various different compilations from different sources over the years, it would be rather confusing, especially when they likewise would show up in all other releases that include those same recordings.
it’s a judgment call, but i feel it’s a sensible one based on those considerations. if there were ever any indication that a single date shown on the release applies individually to all the tracks/recordings (let’s say we know it’s a studio album), then of course it would be a different case.
the other aspect one might point out is that of the renewal of copyrights, but then, i believe those have a set or even predictable number of years of validity to them. eg back in the day, it was 28 yrs renewal up to 56 yrs, though i’m less sure what the current status on copyrights protection is. but i don’t think it’s a situation where copyrights of original sound recordings are just randomly ‘updated’ every time a new compilation is released.
perhaps we could consider each distinct compilation as an aggregated recording of its own, comprising all of those individual tracks/recordings (much as we already do with compiled/composite recordings vs. the component recordings that are compiled within them). so the ℗ year applies to that compilation as if it were just another “recording”; and so that date doesn’t apply to the original individual recordings that are included in it. i hope that makes sense.