Often enough they are just poorly thrown together ROM collections. Often enough the worst ROM available is used. And they have numbers at the beginning of their names to sort them by whatever insane quality metric the company had, so good luck finding anything good.

I’d only buy one where you can supply your own ROMs. They are cheap enough these days.

No idea why Nintendo and Co let them do this. Sometimes they don’t contain games by the big companies. But most of the time they actually do.