I look at it this way
As example someone is "involved" with 4 people. By this I mean either talking or initial dating.
That person has to split their time between work or studies & these 4 people.
Eventually the best of those 4 will get fed up with that person's inability to give them time / put an effort (similar effort) into the relationship so the best will leave & all that person is left with is the dregs that can't get any better themselves.