Wagering is required on all casino sites after depositing. These terms have been kept to prevent money laundering. At least 1x wagering is required to withdraw. But if you want to unlock the Bonus, then you must complete 10-20x wagering. For some games, wagering is required up to 40-100x. So it is better to have a good knowledge about these things before accepting the bonus. And of course, you have to accept this risk.
Money laundering is one reason but another reason is to prevent cheatings from users as well as guests who only want to join casino and wait for opportunitites to cheat.
Additionally, they have to host their marketing programs including bonus programs with very careful designs in order to avoid cheating, as said, and also maximize their chance of getting enough traffics, active bets, and revenue for the casino so that marketing budget spent brings kind of impactful return for their business.
Marketing programs with offers that are too good to be true must be assessed with high cautious approach and too high wagering requirements must be considered as very dangerous offers. Experienced gamblers know which offers are acceptable in risk and good enough in returns so they will more wisely decide to stay outside a program or join a program that looks less risky while good enough for their benefit.