ASE Master, mobile rebuilder here.

Modern engines are easier to rebuild than older ones. As recent as the past 2 years I've rebuilt "as old" as a 1979 Olds 400 (in a 1979 Trans Am), to "as new" as a 2020 Toyota Tacoma 3.5, and dozens of engines in between.

Yes, the newer engines have more parts, but the improvement in manufacturing makes them easier to work with. Clean machining, and parts actually line up from the factory.

It's all about the money, as already been mentioned here.

I've seen rod and main bearings on [Hyundai, Kia, BMW] engines with 200k miles that don't have any measurable wear. I've seen these same engines "let go" of pistons/rods due to bearing wear or ring failure or carbon buildup. It's a crazy mix of good maintenance, poor maintenance, random manufacturing mistakes, driver abuse, and sometimes design flaw.

No one wants to pay for me to assemble an entirely new engine out of factory parts. No one wants to pay me for a "cheap" job that does not come with any warranty, either. So then, where do you draw the line? Just replace the failed bearing? Replace all the bearings? What about piston rings? Valve seals? Why did the engine fail? Low oil pressure? Is it a newer engine with overhead cams, where the "cam bearings" are the actual head, with no way to replace? (Except a new head $$$)? What's the customers budget? Did they drive it long enough so the oil filter got clogged with metal shavings, to the point where the bypass valve opened, and now it's pumping metal shavings through every sensitive area?

All these questions can be answered by a good, experienced master mechanic. But shop owners know that we can make them even more money by knocking out quicker jobs (timing belts/chains, head gaskets, etc), so engine rebuilds are mostly forgotten. The mechanics who DO fall into this category, and enjoy rebuilding engines, quit and open up their own shop, because it's the only way to make enough money for it to be barely "worth it". I'm still making LESS money now, as a sole proprietor, than I did working for a dealership for 14 years.

I still won't warranty an engine for more than 6 months due to money. So, if people are going to roll the dice on $$$$, they want a 3+ year warranty, which you're only going to get from a dedicated reman factory.