If they were good enough, they'd start. Every club would prefer to have cut-price youth doing what Hazard, Sterling, Salah, Alli, Martial etc., do. It saves on transfer fees and wages. But most players don't make it to that level of ability, because they don't have that level of ability. It's rare - but not unheard of, obviously - for an academy, home-grown player to slip through the net for a top 6 club. Even if they're not producing right away, they'll show enough to the scouts and first team coaches to say "this lad has it, let's keep him". But most academy players don't have that for senior level play.
Top players will always get a chance.
The absolute elite – the Sanchos and Sterlings – will get a chance, because most coach's/DoF's judgement is good enough recognise that kind of talent is so special. It's those below that level who likely to have the appetite and skill to step up given an opportunity, but those chances are fewer and fewer.
Think of how many transfers go wrong (Paul Tomkins estimates ~50%, right?) and how many players at PL clubs are bought from foreign markets using limited information. Like, what is Lichtsteiner giving Arsenal that Maitland-Niles isn't giving them?
If PL teams want to produce anything more than 1 player every couple of years, they're going to have to make a greater effort to rely on the lower level of player I mentioned. Then again, perhaps that isn't what they want, because they all have more short-term focuses, which need to be taken into account also.
