here are 3 reasons why:
1) To give "hanger queens" more of a chance. When's the last time you've seen a C.202 or a KI 61 flying around? In my opinion all 4 engine heavy bombers should be perked, as well as all late war fighters (i.e. P-51D, Spit 16, ect.....). also in the case of aicraft that have multiple variants (i.e. BF-109, Spitfire, ect....) the earlist one would be perk free while the next model would be slightly more expensive.
First all, quite a few decent pilots fly higher eny planes, they have as much fighting chance as the pilot has in terms of skill.
I racked up over 15,000 perks in only 1 year just flying 20eny planes or higher, I usually paired them off, C.205 and Ki-84 for example, ones 25 eny and ones 20 eny, next tour P-38 and C-205 etc.
Novice players simply need to get out of the baby rides like the P51D and step into something thats challenging. Problem is if they can barely land 2 kills in a 51D whats chance they will land more then one kill in a C.205? Most likely Zero.
I see no reason to change the eny system or perk more aircrafts, perks are easy to come by for those who practice and learn.