I agree that content descriptions -- both pro and con, or more aptly, things which one person loves another hates -- are helpful in deciding what to read or avoid.
On the other hoof, I don't think that they should be mandatory. Trying to force people to write what you want rarely goes well, and you don't have that right unless you PAY for it. But more importantly, it's harder than it looks. You have to know what the common triggers are, and there are jillions of them. So distill that down to a handful that almost all trauma-aware sites list, and beyond that, you are frankly guessing.
With hardcoded sites, there is an additional issue: the offered warnings can be extremely misleading. If there is no "choose not to warn" option, everyone is screwed. Really. I have personally written stuff that was not exactly rape/dubcon, but which contained issues of impaired consent that I was dead certain would hit the same people who disliked rapefic. I've written stuff about characters who are essentially underage, even if their chronological age was much older. Without freeform descriptions there is no way to indicate that. And the more obligatory something like this becomes, the more rigid the forms tend to become, and the less useful overall.
Consider how close this is to writing blurbs, which is something a majority of writers hate and are bad at. If there are no warnings, people know that. But if there are warnings, readers expect them to be accurate. Getting exactly the right warnings on a piece is challenging, and some writers really stress over that. They shouldn't have to do it if they do not feel that they can do it well. I'm actually good at this, I know my audience well enough to remember some custom triggers for people who have talked about their trauma -- and I still miss things that I have to go back and add later, either because I remembered it or someone asked me to.
Finally, this takes time and energy to do. You have to think over the work, or skim it, and pick out the things that might be troubling; or the highlights in general. You don't get paid extra for that. It's a value-added service for readers, that's all. And some days, you do not have that extra spoon or extra time to do it.
So if trigger warnings become mandatory, or highly pressured expectations, understand that some of the results will be poorly framed ones or stuff that doesn't get posted. Or people just leave them off anyway and the resultant griping makes them feel bad.
Do the best you can with what you have. Don't pick on other people about what they can or can't do. And if the issue is important to you, the best support for that is to read writers who do it like you want -- and quit nagging the others who are staunchly for the other side.
Nothing is ever as simple as it seems.