After all, they are on the market to do the business. Of course, they will have a decent amount of betting volume during the contest. Still, it doesn't worth for $1M Dollars giveaway. I assume, That kind of promo is designed to attract new players to the platform. A lot of players like me will join the contest, at least give it a try. Still, I don't think they will make that much money during the contest. So, yeah. It doesn't make any sense to distribute that much money.
Some of those new people will stay, and keep on gambling, and they may even attract additional people by telling it to their friends if they enjoy it, and that is what marketing is all about. If you have 100 people on your website, reaching to 200 people would be 2x increase, if you have 1k, reaching to 1.1k would be 10%, both had 100 new people but one of them did it a lot easier.
The more people you have the more people you can get. If you spend a million dollars today and not get it back right away, that is fine, surely you need to make some of it back, or at least could afford to lose that, but you will end up with more people and that is the long-term aim.