Because that's never ever been a total usability disaster that just encourages people to enable every one because they don't feel like fighting it. It's also not effective, given how tightly tied mods are to the core API.
When you're talking about security, adding a bunch of config flags for users is never a good idea. Most users aren't going to understand what it does, and like others mentioned, there's too many reasons to turn it on.
This works until a popular mod requires it (for legit reasons) then enabling the option becomes the de-facto standard for people who install mods.