When I go to AutoZone I can buy a part that is guaranteed to work in my car.
No, you can't.
What you CAN do, is buy a part that is "SUPPOSEDLY" compatible with your car, and has a warranty in the event there is a manufacturing defect.
You CANNOT buy ANY PART, ever, that is "guaranteed to work in your car" from any aftermarket part store. They don't ever offer any such guarantee. Yes, that's splitting hairs, but it's a fact, because they CANNOT guarantee that you'll install it properly, or that you even gave them the correct make, model, year and engine size, or even if you did, that you knew the correct production date and anybody who knows anything at all about vehicles knows that every manufacturer pretty much begins a new production year in September, so even if you THINK it's a 2008, it might be a 2009, depending on the production date, and that date might result in VAST differences in compatibility based on design changes, emissions laws, country of origin, changes in engine size options or designs, etc.
They also, OFTEN, have problems with manufacturer's (And their own) parts catalogs being wrong by application. Add to that the fact that sometimes a part is superceded by a newer part that will work in place of the older part in a certain application, but will NOT work in place of that part in a different application, because some aspect of the superceded part design didn't account for the fact that in the older or different application something that was changed was required to have been that way for fitment purposes and now won't work.
Further, we have situations like the Ford F-150 spark plug situation that existed for years where the plugs came apart in two pieces, leaving half of it stuck in the cylinder head, for years, before a tool and method were designed to get around having to pull the cylinder head. In that case, obviously even the dealer couldn't guarantee much of anything even after the parts were redesigned to not do that, since most buyers were taking the newly designed part home and discovering it couldn't be installed because the old part couldn't be removed.
Therefore, there are NO guarantees, as such, in the way you put it. Not even to guarantee that fitment will be accurate. Of course, on the other hand, they WILL guarantee, usually, to take the part back if it doesn't fit or isn't the right part, so long as you haven't attempted to install it but equating that to PC components now you're in a situation where the chances are good you would have no idea whether it was compatible or would even fit until after having attempted or actually installed it and technically in most cases, that voids the warranty for most electronics, even and especially in cars.
Go buy a computer for your car at the part store, and install it, and then try taking it back to the store and telling them it doesn't work. You know what they will do? They tell you, well, either you ordered the wrong part or you need to take it to the dealer to be programmed for your vehicle, and in either case, it won't be warranted. Install a throttle position sensor and take it back. Nope. It was installed. See the marks showing the connector was plugged in? No refunds. Again, technically, based on most part stores corporate guidelines. Now, we know often they still will because many employees don't follow rules well or even look in the box, but it still points to the flaws in the statement that the part will be guaranteed to work in any given vehicle.
To more pointedly address your initial complaint, in addition to what else has been said here, the fact is that if you were around and building systems 30 years ago you would know, without a doubt, that despite the modern computer being VASTLY more complicated and capable than what was around then, they are still incomprehensibly easier to find and install compatible parts for as you don't need to figure out things like (In MOST cases) some custom driver, or the proper serial port or I/O channel, which jumpers must go where, and so on.
30 years ago, a viable network troubleshooter that would automatically tell you what was wrong with the network configuration didn't even realistically EXIST (They did, but they were almost entirely useless in any meaningful way), much less work properly, and now they can and will tell you if it's a DNS problem, if it's a network adapter problem, if it's a routing issue, and so on, which is immensely helpful in narrowing down what part (Or no part) of the network is likely to blame so you at least have somewhere to begin looking AND in many cases can automatically fix the problem itself by self resetting the network adapter, or other hardware, or by requesting a reset of IP adresses etc.
So, things have gotten much better, and a lot more automatic, but things have also become universally more complex in terms of capability, so it might not SEEM that way. Seem being the key here.