I also thought that by getting the bonus, my chance to win would be more so I tried to get the bonus. But in reality, instead of getting the chance to win, I only increase the number of defeats

Besides that, the bonus will always come with the applicable requirements that we must obey so that when we can win, we can withdraw the money. We must be wise in responding to the bonuses the casino gives so as not to use more money to gamble. And if someone can enjoy responsible gambling, he will get pleasure and try to prevent great losses by limiting the use of money to gamble.
That is common but as you have noticed it is a mistake to think this way, bonuses can help to offset some of the losses you have sustained, and for the casinos this is one of their main purposes as they want to give something back to their loyal gamblers.
However if you try to gamble more than what you are used to just to get a bonus, then you are going to find out that instead of the bonus helping you it will play against you as your losses will be higher than if you did not chase the bonus.
I'm agreeing with @ethereumhunter and I believe that those bonuses at some point are only aiding and leading to more of loses than gains and at some points, these casinos have to reduce their terms with regards to bonuses because it seems so impossible to claim these bonuses because it will require you to wager more and in most cases losing more.
@WXA7115 I also agree with you and most time I also think the casino is giving back to their loyal customers but when they do so, they should also try as much as possible not to make it seem difficult to clam these bonuses.