The only logic is that you cant develop youngsters with an inferior team.
Last season Everton U23 had the second youngest team in the division. They were never competitive and lucky to avoid relegation. Talented players suffered from playing on a bad team.
I think Manchester City is the only team in the league that does not regularly play overaged players.
Last week Everton played Manchester United. United have had a shocking start this season and needed a result. So they showed up with a (soon to be) 23 year old goalie that has played close to 100 matches of men's football for teams in League 1 and 2. By far the best player on the pitch and the only reason United got a result from that match. Cheat code.
If Anderson had better offers from elsewhere, I am sure he would have taken them. If he is happy with playing U21 football for another season or two, I dont see why it is a problem to offer him that. As long as he does not block the development of other players.