Though, you must keep in mind welcome bonuses are usually an advertising and market strategy used by casinos which have not yet established themselves very well in the market and they are not as famous and trustworthy as the biggest casinos we all know. Since they need to attract new gamblers for their volume to grow, they opt to offer those welcome bonuses and hope some decent percentage of gamblers will stick to their website even after they have depleted their initial deposit and bonus.
In reality, if a casino is already big, liquid and has a huge volume of bets and gambling in terms of USA dollars or Bitcoin, they do not need to use the tactic of luring gamblers in with welcome bonuses. So, be careful where you deposit your money and whether you are comfortable with the service before doing anything else.
Attracting new clients for a casino is a constant process. It does not matter how big or small the casino is they always need to fight for the new player. A tool like welcome bonuses is the most common method for casinos to attract new clients. Whether the player stays or leaves depends only on the casino on their interface on their lines the number of games support withdrawals and many many other factors. It is a constant and continuous process that depends on a large team. Casinos also have other promotions besides the welcome bonus, and I think if it didn't work for them, they wouldn't do it.