As I mentioned in my original post, my plane can accept three engines, from two different companies.
That's probably because it was specifically designed that way, and those engines are all very similar to one another. Lycoming and Continental have a bunch of engines that are nearly identical, at least when comparing size, power levels, mounting, etc. But you're not going to grab some random engine and toss it in there.
My car also comes with three engine options. Each of those engines is also used in other cars and trucks, of different marques.
Again, that's because those cars were specifically designed that way. The mfrgs wanted the flexibility of being able to offer different engines to customers. Lots of cars have multiple engine options. But you're not going to take a random Chevy and throw in a random Honda engine, they're just far too different. There are some small companies that specialize in making kits for installing engines into cars which weren't designed for them, but these kits are usually quite pricey because they have a bunch of custom-made parts, and that's even when the new engine is from the same mfgr as the old one. You need custom-designed engine mounts, driveaxles, wiring harness adapter, maybe a different hood, etc. And a lot of times, with kits like that, you can't install all the normal equipment, so you might have to go without A/C because there wasn't any room for it.
In long-term essential software especially, it's good not to be locked in to a single supplier.
Yes, and that's why your plane was designed for multiple engines. But not just any engine, only the 2/3 models it was specifically designed for. You can't just grab some random engine (even if it's a similar size and power rating) and toss it in there (though truthfully, assuming it's a typical Cessna-like front-engine plane, it's probably easier than swapping engines in a car).
As for automakers and their suppliers, automakers have *always* used suppliers. Heck, in the early days the automakers didn't even make their cars' bodies! Have you forgotten about "Body by Fisher"?
That isn't done very much in software.
Sure it is. You get a database from one supplier, you get components for it from another supplier, you get a source-control system from yet another supplier, etc. Haven't you heard of "applications"? Even a lot of commercial software contains components from other companies; you just don't see them because it's opaque to you, just like you probably have no idea which company made the headlight assemblies for your car.