There's a lot of good sentiments here. It always boils down to location, price, and whether someone can picture themselves living there. Shops for cars have definitely declined with younger folk, but it is still a niche product, just like a wine cellar, *** dungeon, or modern decor is.
Surely this opinion will ruffle some feathers here on GJ:
The biggest factor I see in why people don't want to buy a house with a shop, is the quality of the entire property. Too often the focus is just put on the shop and the house is left just "good enough" with no updated kitchen, bathrooms, and very little space. But hey, there's a HUGE shop out back dwarfing the house that is fully equipped, epoxy coated, mini-splitted, with a bar and office. Sure, they'd think that is bonus, but the priority in shelling out the dough lies with the house.
The other factor, to be realistic, is most people with a "gitterdun" attitude just plain don't have the skill or care for design (or prioritize functionality over it). As examples you see garages tacked on with odd roofs, or detached placed on the lot to allow for an RV or trailer turnaround so the overhead door and driveway dominates the house. Multiple buildings just placed what seems as random. Nice brick house, but corrugated metal garage blocking the view of the backyard. All that's fine and dandy, but if you really want to sell down the road or are concerned with your ROI, you would have put some face brick on the front, a decorative dormer, wouldn't have built it where a back patio should be, etc. Even in rural areas (not talking ag) where regulations are not as strict, more successful sales I see are ones that put in as much thought and money as they do with the house - everything is consistent. It's no different than a buyer passing on a house that has an odd layout or crappy workmanship, the shop is just an extension of that house mindset.
Granted the funds available certainly determines the level of house and shop, and sometimes you have to sacrifice the house to get a decent shop. But if someone looking to buy has X dollars, they would spend that on a better house where they can add a shop later, rather than spend their X on a lesser house that already has a shop. Look at 90% of GJ threads, it's about building a new shop where they already live. So the garage lust is there, just very few people will buy based solely on their lust over your particular garage you built for your needs.