Maybe some of it is born of past experience, advertising and expectation?
* Air travel, by and large, getting safer, even as it expands.
* Flights timed (and measured) to the minute, punctuality very important.
* Despatch reliability, comfort, noise, etc. all improving as fleets modernise.
* People are told (and expect) that they will get there on time, on price.
* Flying is presented as a seamless alternative to other means.
So pressure from customers as well as management. Not only that, it is possible to take questionable actions (in retrospect), on the basis that it's good for your clients and everyone who's a team player wants that, don't they? Part of what we see might not be people caving in to threats from the airline(s) but from a genuine desire to do what appears to be the right thing as a team that's 'onside'.
Maybe after you've taken "plog" fuel for 20 sectors, it becomes more difficult to justify extra as it feels "wrong" somehow? When I first got my command, the boss *told* me not to worry about fuel and that he'd be happier knowing I wasn't being distracted by it in the early stages. Later, I had a period of benign weather and took almost a month of minimum fuel (with no problems) then did a trip to the far east - the FOs both wanted more fuel and my initial reaction was to think "that's spoiling my record" and shortly afterwards "thank **** someone's thinking straight!". I was a bit shocked that I'd managed to lapse into minimum = good, even when it wasn't; needless to say we took a fair bit extra which we needed most of at the other end.
I think other posters are near the mark when they point to regulation, company ethos, training and "normalisation of deviance" being munged up together to make an unholy brew. There are multiple reasons and it is somewhat unfair to single out any person or organisation. FR get a lot stick but considering the size of their operation their incident rate is probably pretty low.