Read an interesting comment on reddit about it, regarding the different categories of academies and why Brentford doesn't have one. From memory from 2000 to like 2011 there was the 90 min rule for academies, which meant academies could only sign players that lived within 90 minutes of the club. That led to crap academies which so was scrapped, now there's different tiers of academies like United's which I'd assume would be tier 1. But the rules currently allow tier 1 academies to poach players from other academies for peanuts, the reddit comment gave some examples, which basically means that smaller clubs like Brentford can have their best talents poached, leaving them running academies that don't really benefit them. Especially being in London, with so many other academies for players to go to. So they instead focus on recruiting well from other leagues.