I've brought up this issue in mathematical circles before and have encountered incredible resistance. The culture is very deeply entrenched, and the current stakeholders are not going to allow it to be changed.
Just to briefly establish that I have some qualifications here, while I do not have a PhD and am not nor have I ever been a professional mathematician, I did have a lot of success in mathematics as an undergraduate, publishing research in the intersection of algebraic topology, differential geometry, and analysis with a professor at an Ivy League university.
I was myself a practitioner of the school of It Is Clear That..., until I realized that my mentor for the above research, a famed professor and former Putnam Fellow who even has inequalities named after himself, didn't follow some of the stuff I'd written in some notes I showed him. And if he didn't follow it, then I knew that meant I probably didn't understand it as well as I thought I did. It wasn't a big surprise to me then that, after a few weeks of effort, I was completely unable to take my approach further. His approach, of course, worked.
He represents one of few mathematicians I've personally known who emphasized clarity just as much as correctness and technical depth, and he had no desire to appear smart. But at this point in time he already had his career mostly behind him. There was nobody left for him to impress.
Part of the problem, ultimately, is that phrases like "it is trivial that..." do serve a purpose when used appropriately. As a result, it's hard to argue that they should categorically be excluded from mathematical writing. But that then opens them up to abuse, and in a culture so obsessed with appearing smart, that abuse can be quite extreme sometimes. Because instances of abuse are often motivated out of appearing smart rather than legitimate economical concerns of space, bringing up the issue with the author amounts to a moral accusation against them, which makes their defense of the phrases even more impassioned. They're not going to admit that they're just trying to puff up their ego.
The worst part is that, in some circles, if you criticize this sort of writing you'll just be greeted by a bunch of people who are proud to exclaim that, of course, they are smart enough to fill in the blanks, and if only you were too then you surely couldn't possibly still have any objections to it.
For me, transitioning from mathematics to software development was a huge breath of fresh air, as the culture is (generally) the exact opposite. Nearly every good programmer I've met has valued clarity and maintainability equally alongside correctness.