Difference between us and them is we had a poor academy set up till MB came in. Those players they play have generally had 2 full years out on loan and been in their academy for a long time.
I am also not sure where you are getting the clubs from who are doing so well and playing a bunch of U21s, if you look at the top 6 you've got:
Chelsea
Reece James (21)
Mason Mount (22)
Trevor Chalboah (22) few appearances
Malang Sarr (22) sub
Hudson-Odoi (21) Sub appearances
Not sure if you are counting Loftus-Cheek in your thinking but he is 25
Scum
Mason Greenwood (20)
Jadon Sancho (21) Doesn't get a kick and is being horribly managed
Marabchester City
Phil Foden (21) 2 years of intense focus with no loan out and now look at how good he is
Liverpool
Alexander-Arnold (23) Old
Curtis Jones (21) Gets games when others are injured
West Ham
Ben Johnson (21)
Declan Rice (22)
Arsenal
Smith-Rowe (21)
Tavares (21)
Saka (20)
Odegaard (22)
Lokonga (22)
I was going to do likes of Spurs but whats the point they don't play any kids whatsoever
When you look at that and compare it to our 'good' youngster we have:
Meslier (21)
Struijk (22)
Shackleton (22)
Summerville (20)
Gelhardt (19)
Drameh (19)
Greenwood (19)
Cresswell (19)
Out of those Meslier, Struijk and Shackleton prior to his injury were playing regularly so we have as many 22 or younger players in the squad if not more than most of the teams up the top currently.
Whilst I agree I'd like to see them more I am also mindful of the fact that though Gelhardt did come on and change the game vs Wolves he has also come on and not had much impact (which is not his fault, because he is a young kid) so that always needs to be kept in mind. Likes of Summerville and Gelhardt need to be given these short opportunities against tired defenses to build their skills and understanding then build up to the first team picture as regular starters.
When you also look at the ages of players they're playing from what I know none of them are playing kids under 20 which is the age of most of what you'd call our promising players. So although some may be playing youngsters, they aren't like ours and some of those clubs have paid in the tens of millions for these kids.
I agree I'd like to see more of the youngsters but I also trust in our club to manage these kids properly, I remember they days I'd be sat there wondering why people were so desperate for Dom Poleon to play like he was the next big thing so sometimes we don't see what they're seeing on the training ground.
Look at how Pep handled Foden people were clamouring for him to be sent out on loan, given more first team minutes and he was held off for 18 months and now he is probably the most exciting young player in the country.