You very much can, people just ride too hard. Then go all shocked pikachu when they are tired.
I used to think it was a super human power that pros could ride 250 miles a week but then I got to know a cat 1 on our team. He would ride like 20-25 hours a week. Most of his riding was a lot of noodling - serious noodling, almost falling over sideways from going so slow level of noodling.
It just seems fast to us when guys with 400 watt FTPs are nooding around at 180-200 watts.
I think most of the likely reasons have been covered here already, but I haven’t heard anyone talk about heat. I know I start to fall off when the temperature goes up significantly. Possibly one more thing to consider.
Bingo. When I moved to 16+ hours a week for a couple of years, I started my endurance rides at like 50% of FTP after suggestion from my coach when my fatigue went through the roof. Couple months of that, and I was on track.
I think there are two parts to this: 1) more volume is more aerobic stimulus and at relatively low intensity, TIME matters more than intensity. As you mentioned, a lot of riders aren’t going to start out being very happy doing 145W for 5 hours. It wasn’t great for me at the start. But within a few weeks that was becoming 160W and I was moving closer to what I was used to doing as time went on, and I was fresh for my intervals… lo and behold, I started having good interval workouts! (2) When you’re dealing with relatively higher FTPs, even endurance riding is a very high metabolic load, so using strict “zones” may not work out very well because let’s face it: 55% of 250W is not the same metabolic load as 55% of 350W or more.
I started seeing some progress on high volume when I dropped my endurance load to like 50% of my FTP, and trained there a lot. And then FTP moved up to my normals and I was able to sustain it longer and progress. I was just beating myself up riding 14 hours at 65% because Coggan said that was good and then adding interval work. (Not his fault, just saying).
When you start adding volume, you should do it in a stepwise manner and back off the intensity while your body adjusts… or you’re gonna have a bad time!