Until the late 60’s it was a grownups world! Slowly it started become about what the kids want - until eventually that mentality took over completely.
I’ve worked at my place of employment for 21 years. When I started there, the (what was then still known as) “Christmas” party (now a “Holiday” party) was a time when everyone seriously dressed up for this big annual event.
Nowadays, this is only a memory...now employees just wear casual stuff, whatever they had on that day. The ones that would dress up back in the day 20 years ago have since retired and all we’re left with is a bunch of snotnosed kids.
I think you’ve helped answer the question, “What happened?”
I can still remember the first dress I wore to my first adult (as in I had a professional job) Christmas party. Red taffeta, nipped waist, beautiful big ruffled collar. This was only 23 years ago. Now, you’re right, dress up is wearing clean jeans.