"the grass is always greener"
my guess is, in each other "more desirable" country, there is a forum with someone saying "is it time to leave, X looks better"
For example, Oz and NZ are often desirable countries for ppl to go to, if you have the points, I know ppl who went there years ago, have no desire to come back, I know ppl who went there, after a year or so came back. I also know Australians and New Zealanders who have come here with no desire to go back.
I also know Polish, German, and other Europeans who have settled here, and despite Brexit, are happy here still. Some came here as a couple, and have since had children here, others came here single, and met their partners here and have started families, so going back would be a massive undertaking, and maybe they have discussed going back, but they don't seem in any rush to.
However that's anecdotal, the next poster may say they've lost 1/2 their workforce as they've gone home, and there were stories that a lot of manual workers, fruit pickers etc had returned home, but I wonder if there's a lot of Brits abroad who have come back for similar reasons?
Better the devil you know?