Most likely because this was a star ship (or space station) with a limited number of personnel, all of whom have fixed duties that need to be done. You simply can't afford to waste your time away in holodecks.

The people we saw on screen most of the time also held important positions on the ship (especially the bridge, or engineering) and you can't expect them to just waste significant chunks of time.

Also, don't forget that these people actually like their jobs. They got there because they sincerely wanted to, out of personal interest and drive, and not because of societal pressures like in our present world. They already figured out universal basic income and are living in an advanced self-sufficient society, so they don't even need a job to earn money or live a decent life - these people are doing their jobs because of their pure, raw passion for that field.

Also, holodecks are limited in number. Voyager had two, and during one episode where the plot point was that they were in an area of space with literally nothing, the holodecks were in such high demand they had to schedule time there so everybody got a bit each. With Voyager having 150~ people onboard, I can easily imagine that sucking. The Enterprise had more holodecks (4-6~?), but with around 1000 people onboard, if they were in the same situation of there being nothing to do, the Holodecks would probably have been equally crowded.