Main reason: Too frequent updates with little noticeable improvement is a hassle.
There is a definite case for those to be a silent install though.
(And the real push-back will probably come if a silent update accidentally breaks things, so I guess there's extra incentive to make sure that doesn't happen.)
The thing I hate about silent updates is that if they cause something to break, I have no indication that a browser update is a possible culprit. If I am informed of an update and then find that something has stopped working, I at least have a starting point for resolving the issue.
Main reason I don't like using Firefox: It runs an update every friggen time I start it.