1) People seek purpose in their jobs. The jobs that provide this usually require an investment of time.

2) People seek status. Status comes from relative wealth and intelligence, which requires an investment of time.

3) People usually don't have the choice to trade money for time. You usually can't say, "I'm going to work 5 hours less per week and in return take a smaller salary". Hourly workers can (sort of) do this, but they're the ones who usually can't afford to do this.

All that said, there still are plenty of opportunities to exchange money for more time and people (IMO irrationally) pass these opportunities up. Ex. paying for food instead of taking the time to cook, clean, shop. Ex. paying someone to clean your house. I'm not really sure what the explanation for this is.