But the point is that it actually takes very little time in the gym to build and maintain muscles.
If your routine involves the squat and you progressively load it each week, yes, you'll build muscle. That single exercise can easily suck up 15 minutes of your session though, and the time investment is usually greater the longer you've been training. And typically people are interested in aesthetics in their upper body. You're not going to build your arms by doing a squat. So, again: it depends on your goals. If your idea of fitness is having strong legs, glutes, etc, then sure, you can target that and be successful in one hour. If you have bigger aspirations, the story is different.
Another thing to keep in mind is that there is a technique to certain compound (free weight) movements. A lot of people don't start their fitness journey that way (and some have good reasons for doing this). If you start using machines that are more targeted, then you will easily blow past the hour mark.