Nowadays, many parents, especially Asian ones, tend to because as long as they do so, they certainly will be successful. I think there are many advantages to this philosophy.
It is undeniable that there are many benefits to telling children so. The first reason, obvious as it is, is that this will encourage children to become much more diligent. Self-discipline is to develop for almost anyone, and telling children that success awaits them if they work hard incentivizes them to become self-disciplined. As a result, they stand a better chance of being successful since they are more goal-oriented and determined. Another reason is that these children will exhibit appreciation for hard-working people. They will perceive these people as also wanting to be successful like them and thus appreciate their diligence. Therefore, these children would be more grateful and humble towards other people.
However, there are certain disadvantages to teaching children to expect success once they have worked hard. The first disadvantage is that these children can ignore other factors for success, such as natural talent, soft skills, attitude, or connections. For example, a child might be so engrossed in one's studies that one neglects to make friends, which might serve as crucial connections many years later. The second disadvantage is that children will show if this philosophy fails them. For example, when children study hard for their university entrance exam but cannot get into their dream school, they will blame themselves for not working hard enough. In the long run, this will be mentally exhausting for them.
In conclusion, there are both advantages and disadvantages to telling children that once they work hard, success will come. Generally, I think parents should encourage their children to work hard for their success, but diligence should not be the only factor leading to success.