I'm thinking out loud, as I never really thought about it in that way.
I think it is a bit of institutional inertia. A great deal of private sector organizations structure what they do around the timeline. You're talking about a rather disruptive change to the way we've always done things. I don't see any internal institutional barrier to doing that, but I'm certainly not an expert on it.
You're absolutely right, though: this idea is brought up with exactly the same rationale over and over again, and for whatever reason, it isn't being done.