What is the reason for losing every bet?

Gamblers use empiricism to sum up the truth that "gambling for a long time will lose", but not many people can understand the mathematical principle.

Gambling for a long time is bound to lose. Many people believe that the fundamental reason is that "bookmakers" use various methods to set up gambling or even cheat, which makes the gambling probability biased towards bookmakers, which is not good for gamblers. In the long run, gamblers will not be able to do it. This is a wrong understanding. Gamblers have lost all their money because they have been gambling for a long time. As long as the probability of winning the bet is less than or equal to 50%, the bet will be lost. It is easy to understand that if the probability is less than 50%, it will be lost for a long time, but if it is equal to 50%, will it be lost? This is not very intuitive, maybe not intuitive, but it can be proved by simple mathematics.