Its because of the scam.
You see, when you have billions of coins, chances are the selling price is in single satoshi digits. So with large pre-mines, insta-mines, the devs can sell the coins, even at 1 satoshi, and still profit.
Then rinse/repeat with another billion coin.
This^, the mass majority of coins with such high max supplies have been, are, and are going to be, scams. This because it simply looks "better" if a dev announces he's going to have a 10% premine/instamine out of a coin that has a 100billion max coin supply and more likely to actually gather a small community, rather than saying he's gonna take a 10% premine/instamine out of a coin that only has a 10million max coin supply(which would instantly put the scamdev on the red alert to most people).
Coins with stupidly large supplies don't quite make sense, as Bitcon itself is divisible to 8 places and more, making having a large coin supply irrelevant.