There is a difference, though, between quality time and quantity of time. I may be able to have a very meaningful, enjoyable, and deeply satisfying, loving relationship with someone I don't physically get to see very often, which leaves time to spend with others. Or I can have FWBs whom I care about a lot but see more often. <shrug> Love comes in various shades and hues, it can be expressed less often with some people and more often with others, and still work out quite well for all involved.
Seems like you're judging a bit - and that's okay, but... do you view people who have more lovers than you feel you yourself can handle as not really giving their all to a relationship? You seem to be questioning the quality of the time they spend with people they love, just because time is limited.