My guess would be this is a holdover, from when engines had a quality level. Items with a quality level can’t stack. Now that engines have no quality, it could be worth re-examining this. On the other hand, engines are certainly one of the rarer items in the game, and having one is enough to do whatever you’re doing with it. So maybe they’re non-stackable to reflect that this is an ‘important’ item that should take up its own space.