Typically because of parts. They take time to refurbish and procure of it needs replacement or make if the part no longer exists.
Discussion
So logistics and money won't make it faster for the first time and then noone invests into faster infrastructure.
Right. More money only shaves off hours, maybe days.
Even the manufacturer themselves, like Lamborghini for example, it took them a couple of years to restore a car they made 60 years ago.