Because most people younger than that seem too young and green and don't have the gravitas for the part.
Yes, we need to think long term. But if that means most of the movies are going to be where they are too young, and only by a future movie in 10 years time they'll finally start to look right, then that is probably not the right approach either.
I think someone in their late 30s or 40s is fine.
For James Bond, he has always been about that age. If they go too young with him, he just doesn't have a certain distinguished feel yet. I think that's the same with Reed. He has to seem like a man, not a boy.