The magic cards that cost thousands only cost so much because they aren't in print anymore. Ones with that kind of price point are collector's items and aren't even legal to use in the formats that are commonly played
But the booster packs that contains those cards are still available for sell. So why WotC isn't the one profiting off of it, private businesses still are selling them and some people do open them hoping for one of those rare cards. While it isn't a price point a normal child can afford, I don't see why it is different given that the items are still being actively sold.
Well the randomization is actually part of the product for a lot of people, boosters are typically used for sealed/draft games for example.
Secondly the booster packs that contain cards that cost thousands aren't still for sale, at least not from Wizards, though you might still find a few unopened ones around. Some of the cards available in current standard legal boosters might cost quite a bit, but not thousands
