Because they don't have to. Looking for new investors will just lessen their share in the total profit of the website. They're already earning money and they want to keep it to themselves. Most gambling sites also have rich investors right from the start. That's how they have established everything, by getting a help from a rich investor. They don't need to look for some more online because they're website is already well funded.
Then why leave the opportunity to invest if basically everything is established?
I think that any external investment is good for the casino. Even due to the fact that increases Bankroll - which attracts more players with a bigger wallet.