And when you do restart, every single open tab will/might be able to load everything, unblocked and unfiltered, in the period between Chrome starting, and starting the extension.
For some people it is not just about annoying ads; for those people paid or unpaid work (or something else, I'm not here to judge) takes them to sites where you'd rather not be surfing with js enabled.
Remember: Client-side JS is a way for whoever controls the server side to execute code on your machine. Disabling JS instantly removes whole classes of nasty exploits.
On the other hand, I don't want random extensions, which could be misbehaving or poorly coded, to be able to indefinitely delay the browser's launch, even if it comes at the cost of one ad making it through. Imagine having dozens of extensions and trying to figure out which one is slowing down your launch because there's a bug.