Nutrition, sleep and work stress are definitely factors but you are forgetting what is by far the most important one - Genetics. And I did say there was a huge variation among individuals. If you look at the last chart in the first link I posted there is a variation of around 1.5 w/Kg between the highest and lowest at pretty much all tss levels.
While I know there are plenty of people who just don’t want to work very hard and like to pretend it’s bad genetics there are also plenty of genetically blessed people who like to act like everyone else is lazy.
Personally I have been training pretty consistently for the last ten years including two ironman builds (up to 18 weekly hours) and several half-ironman builds (up to 15 weekly hours). I have added about 50 watts to my FTP over the last ten years pretty steadily. Last year I was around 250 at 80 kg (3.125 w/kg) using the sufferfest plan.
I had hoped for some amazing gains with the trainerroad plans but so far not much. To be fair I have switched to a paleo type diet this month and lost 11 pounds so I think the caloric deficit has hampered any gains. I am in between the Low and Mid volume plans, currently halfway through SSB2.
On local club rides I am usually somewhat close to the front of the pack and at the local cat 4/5 training crit I can hang with the pack the whole race but not really make any kind of serious attack.