Who said that 35-40 hours is optimal? How can you even quantify it considering every job is different. I've had jobs where 50% was brainless support so i'd work 10-12 hr days and my brain was totally fine to keep going. Some jobs come with inevitable social BS that will take 50% of your time anyway.
The problem is that management and developers have different incentives. Managers [in more cases then not] are pushed by executives to justify their worth and deliver results.
Developers want to create, but also don't care as much about the bottom line. They want to have a life outside of work.
It's that simple. Let's not go pretending like anything beyond 40 hours takes 10x time to get done because it simply doesn't. It's just an empty thing to say like "you should give me a raise because of inflation".
There are plenty of people working their own startups doing 80-100 hr weeks and getting shit ton done. Does it mean that you should do this at a corporation? Maybe not, maybe yes. It all depends on the trade offs either side is willing to make. Managers will of course be incentivized to push for more productivity per employee.
Places such as Basecamp take a stance against this incentive gap. They're actively working at making employees take more vacation, sleep better, and have a better work life balance. This must improve employee morale and retention. No idea what it does to bottom line or employee paychecks.
As a manger I've had people that worked 40- hrs and delivered amazing results. I'd never even think about caring how many hours they've put in. Then there were others that worked more but delivered a mound of technical debt or nothing at all. They were usually the ones most vocal about working too many hours. There are very few people out there that can come in, get straight to work, and deliver quality. Simply making a blank statement that they will do more by working less is silly.