It used to be the case that you could "bank" unused vacation-- often indefinitely. When I started with HP in '92, there were engineers that saved up literal months of vacation time, and would then take it all at once to go spearfishing in the Bahamas or what-have-you.

Then when the .com bust happened, everybody started tightening their balance sheets, and huge piles of unused vacation time was an easy liability to get rid of. So they reduced the max vacation accumulation to 3 or 4 weeks over your yearly accrual in a year (but left you what you had, so you wouldn't gain new vacation if you were at the cap, but you wouldn't lose it either).

Then they tightened further, so you'd only accumulate 2 weeks, then 1 week over your yearly allotment.

It's possible that habits built when vacation was easy to hold on to die hard.