The Pacific Locomotive Association has had 10-year restorations. Here's how it was done in 12 days in the heyday of British steam.[1] Of course, the repair works had 6,000 people.
Who were also experts working full time, with plenty of practice with that specific model, and loads of specialized equipment.
vs. a handful of hobbyists.
Sometimes when you look at old things, it can be shocking how “sloppy” the workmanship is, but you have to realize that they were not trying to get perfect paint jobs or smooth castings. They were trying to make money.
However an even higher level of quality is the pros that are called in to do one off custom projects for wealthy customers that pay by the hour and don’t care what it costs.
Ultimately, the perfection of the work is basically a tradeoff between time and quality- and the pros produce a lot more quality per unit time. But it is amazing how a really careful amateur can still do impressive looking work if they just go really really slow.