The apostles didn't say that the work of serving tables was beneath them. They considered it, evidently. They saw the problem, and their instinctive response was to get involved and deal with this difficulty. Then they stepped back and realized that in order to do that, they would have to neglect something else. The church would have more blessing if other saints could step up and fill this role, and they themselves could continue to fill their own assignment.
Division of labor is for the good of everyone.
Lastly, let me ask you to consider the qualifications that are mentioned for these seven who were going to solve the problem of distributing food to widows. It first says that they were to be full of the Holy Spirit, which means they were supposed to be mature. A person in whom the Holy Spirit has control, who has learned to walk with God long enough that they respond quickly and easily to the Spirit's promptings, is not concerned about his or her ego anymore. You probably know that there are some people who will volunteer to get involved because they want recognition, or because they want to solve their own problems by meddling with someone else's. But the apostles said that they wanted people who would lead the church into service to be mature enough that they would listen to the Spirit, to be able to answer the call of God, not the call of their own ego.