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.
At times too much options also helps because out of those say you played 10 and one of them might become your favourite and you would want to play more of it. This will make the sites richer as more you play higher the chances of you losing it than vice versa. Every user who comes to site might want to play some or other looking the individual choice.