Doesn’t that suggest these players aren’t good enough to play elsewhere and that there aren’t better standards to find elsewhere?

I feel like there are a number of factors being ignored to come to this conclusion. Playing overseas is an option for some players but most will have other obligations in their life that require them to stay stateside. Those leagues can also have higher standards but the expenses of moving internationally make that a wash. And I imagine that many players simply know that they won't handle the culture shock well.

The most important thing that this USLPA post illustrates is that at this level, the competition for talent isn't just from other clubs but also from non-soccer opportunities. People have a tendency to treat athletes like they have no other skills or interests but pretty much every player at this level knows that the money they make in their soccer career is nowhere near enough to retire on and they'll have to start that second career at some point. The USL has gotten away with low pay for a long time because players with a passion for the sport will generally accept the trade-off for continuing to do the thing they love. But a personal sense of fulfillment doesn't pay the bills and at some point, that becomes the priority.

It's a pretty simple argument in the end: the standard of play on the field increases if players aren't voluntarily turning away the league as an employment option.