Well, it is obvious. In a Casino owner's POV, they already are paying for the hosting and domain of the site, why not take full advantage of it by deploying hundreds of slots. I am sure they also got discounts on getting those huge numbers of slots games. That is like offering too much but paying less. Though they can categorize these slots in terms of creators so that it would be easy for players to track which is which.
And also to catch the attention of players. As every player has their own taste, with so much selection, they have the chance of capturing a lot of players. These players will somehow be bored if only few selection is available in the casino. So that's the trick also by these casinos, offer as many as possible so players will lose track of their time. The longer they play, the better for the casino.
If someone keep gambles without tracking their balance then they will be on the lose for sure at the end of the day because in slots there are high chances to hit winning streaks but if we keep betting we may forget about the losts and just keep chasing the lost wins and that is how the gambling layout will be since it is the profitable one for them.