Myself, my dad and my uncle are all pretty mechanical, Uncle runs a welding/machining shop, all self taught on machining with a knack for fixing things others cant. Grandpa was pretty good, and had some other close family that was extremely good with welding and mechanical work, he figured out how to run 2 steam engines on 1 threshing machine, and afaik his home built track roller welder is still at Cat in Omaha being used. 

Rebuilding engines & transmissions,  welding, upgrading or changing equipment to fit our needs, pretty decent with electrical. Both dad and I have degrees in automotive mechanics and a/c repair. Hes not great with electrical stuff so he lets me do it, but neither of us are worth a damn at 120v stuff or carpentry.  The only things we really dont do are tire repairs and rebuilding starters and alternators, not that we cant but a guy can only do so much. 

I think its a personal thing, some people have a better knack than others, know a guy who has spent his whole life on dozers, scrapers, graders etc. Hes about 70 now, hes never been able to do more than tighten a few bolts, maybe change a belt or something like that. If it were me or dad, wed know the thing inside out but hes always been that way. But he can look at piece of dirt and tell you what needs filled or cut and get it damn good without ever using a transit. 

I do think that some of it is going away, or at least changing, to being able to fix stuff using computers, and just like our cars, engines last much longer than they did, dad talks of overhauling the M every year maybe every other if they were lucky. And its harder to work on stuff than the past.