You state this as if it were objective truth, but it isn’t. Many Eastern Catholics and Eastern Orthodox would strongly disagree, and unlike you, they would do so on the basis of actual experience with married priests. Whether or not one a married priest can serve both his parish and his family well likely depends on several things, such as his own personal strengths and weaknesses, the degree of support of his wife, the size of the parish he serves, the support of parishioners, etc.
If you’re looking at a scenario in which you have a large parish served by a married priest with young children, and he is the only priest serving that parish, then it would be very difficult for him to serve both the parish and his family well. Some people use this scenario as the basis for the claim that one can’t do both jobs well, as if every priest is the lone priest serving a large parish. However, not every parish is large, not every large parish is served by only one priest, not every married priest has young children, and so on.