This isn't going to make sense to anyone who even begins to consider keeping track of their cards. They must have something like autopayment going on their credit cards to avoid them turning into a Superfund site.
Partially, I have to think that you get so many credit cards because they generally lack annual fees but the affinity of the card provides additional benefits with a certain merchant or type of transaction, ie, something like store charge cards give you an additional earn rate of points.
Partially, there's also the gamesmanship of getting the best interest rate, lowest balance transfer fee, or premium for opening a card. If you get something like $100 for opening a new card, and you do that 1,500 times, that's $150,000.
Whoever is doing this isn't losing their shirt with Credit Cards; they've figured out how to game the system. That could mean they just have a giant pile of
cards like this one.
And the upshot of this is that they may well be able to do things like use the million+ in credit to finance something up to a home or car, then balance transfer it around without ever paying interest on it, while knocking down their balance. If that sounds extreme, just remember that the fees are generally lower than the prime rate (3.5%).
There are uses and benefits for this much credit line; but a few mistakes and whatever advantage gained relative to more conventional options is likely lost.