There's obviously no one answer, but I've always got the feeling England are extremely cautious in their selection process and are just unwilling to take calculated risks. It's fine being pragmatic to a certain extent when you have a settled XI (or squad of players) and are consistently winning tests macthes, but sometimes you just have to go with your gut and make bold selections. Liam Livingstone being a prime example of a player who should absolutely 100% have been in this Ashes squad
I think England coaches are probably good at identifying talent, and funnelling them through the relevant player pathways (England age groups, Lions etc) but have no idea how to properly develop certain players to equip them to deal with the physical, mental and psychological challenges which are unique to test cricket.
There are a host of fast bowlers in the county system in their early-mid 20s who just haven't made the breakthrough to the full team, and the evidence is certainly substantial that it isn't coincidental, and is endemic amongst the county game
A few names include Reece Topley, Saqib Mahmood, George Garton, Ben Coad, Sam Cook, Matthew Fisher, Jamie Overton and Brydon Carse. Between them they haven't played a single test match and the majority haven't been involved in a wider squad
There is also a generation slightly older including Lewis Gregory, Jamie Porter, Ed Barnard and Tom Bailey who are all excellent county cricketers and probably deserved a chance in the last couple of summers. Gregory and Porter especially