We loan them because we have no role for them and they won't get meaningful minutes here, and there is no market to sell them.
The reason we don't want to keep them or why it's bad/dumb to keep them is because it is better to have a smaller, "cleaner" squad. Of course, you need depth. But you need the right kind of depth, the right amount of depth, and in the right positions. What use is it having 3 left backs? One of them will quite simply never play. There is no use in keeping a senior player in the squad in case there are serious injuries, to then be squad players in that instance.
My logic is always this - if you aren't in the first 11 or 2nd 11 as a senior player, then you are a wasted spot and a spot that is more likely to bring down training levels, breed unhappiness and disgruntlement in the group than actually contribute anything.
Our current starting 11:
De Gea
Dalit Maguire Martinez Shaw
Fred
Eriksen Bruno
Sancho Martial Rashford
Backup 11:
Heaton
AWB Varane Lindelof Malacia
Garner
McTominay Donny
Amad Ronaldo Elanga
To fill out the squad, usually a few youngsters are good who can hop between the youth team and starters. But it depends then on what's best for them whether they stay or are loaned and if they can actually contribute. Garnacho, Hannibal, Zidane Iqbal, Savage, Laird, Shoretire make up that group right now.
To me, our goalkeeper, back 4 and midfield 3 for both starters and backup 11 is pretty much set, anyone that comes in will need someone else sold. Anyone not listed in the 2 lineups above can easily be cut from the squad with 0 negative impact, and we'd both get some money for them but more importantly, not have a bloated squad which is bad for morale.