Maybe the delta just isn't enough to matter? Or maybe people aren't willing to pay for it.
Remember we’re still pretty early in the history of aviation.
Or when I leave from Boston to go to the San Francisco, and we leave an hour late but we still arrive on time, it's because they were able to go faster. We certainly have the tech to go faster.
So why can't I buy a BOS->SFO flight that is one hour shorter for more money? Probably because of a lack of willingness to pay.
Catching favorable winds and burning more fuel. It is in the airlines best interest to have the plane in position for the next flight, so they will burn the fuel when they need to. However, committing to a tighter schedule would cause a lot of problems if they were late too often, kinds of problems that means they would make less money than with the current schedule.
There is always a price where this isn't the case. My overall point is that that price is still too high and people aren't willing to pay, and we don't really know if that's the case (but maybe the airlines probably do).
This is similar. 3.5 hours vs 7 hours is a pretty good difference.
You can take a 3.5 hours flight in the morning and have energy to see a city the whole day after that. Maybe not after a 7 hour flight unless you are a pretty experienced and motivated traveler who can sleep the entire flight and have the mental energy to enjoy new things after that.