IMO UEFA, FIFA etc. aren’t even the problem.
The clubs are. They’re the ones that pick the players in those matches. They could easily use more of their U21 players and rotate more (would probably be better for youth development for them to get more time in first team games too!) but they don’t. They only use them when first team players are out injured and then whinge about it.
Why should they though? Why should the popular teams share their ticket revenue, sponsorships or merchandise sales?
That just discourages teams from actually investing in the team to improve.
Not to mention that’d would make it near impossible for promoted Serie B teams to compete and stay up.