In the "old" days, wheel bearings were very easy to service, and most shops suggested an annual re-greasing and adjustment. Today, many bearings are not easy to service, and doing the job right will cost more than many people want to spend.
As a result, for folks who have a wheel worth less than $200, and the bike shop is asking $50 to regrease the bearings, it is tempting to simply ride that wheel until it has obvious problems.
I have a pair of touring wheels from 1984 that have never been greased. The shop says they have "odd" seals and impossible to replace sealed bearings...they don't want to fool with the wheels. After 25 years of moderate use, the wheels still run smooth enough, so why mess with them?