As a point of comparison, Microsoft ships its OS across a variety of manufacturers and largely keeps it maintained across them (give or take some exceptions like enterprise environments & the like).
Even unlocked Android phones purchased independently of carriers have inconsistent lengths of support, so it doesn’t seem to be entirely a result of carriers, so…What happened here?
It all boils down to the basic difference between the two Operating Systems.
Android, or rather The AOSP (the Android Open Source Project) is an open source operating system. That means that it’s openly accessible to all, open to tinkering and tweaking. This is both the best and the worst part about Android.
Best, because it means people like you and I can create custom versions of Android if we want (look up custom ROMs). Plus we can look at the code and see if there’s anything we like or don’t like. It’s great from a safety standpoint.
Worst, because this means that it needs to be optimised by manufacturers for each and every device they make. Since AOSP is a general OS, they need to make sure it works with the hardware in their device, with their camera and their processor. That takes considerable effort, although if they chose to focus on a select few devices every year, they’d be able to do so. However, given the race to sell as many phones as possible, it becomes clear why they don’t update lower end devices (which is weird considering the fact that those lower end devices are the highest selling devices).
Windows, on the other hand, is a closed source operating system, meaning Microsoft makes the operating system and distributes it. It can be modified, and various drivers can be installed, but that’s about it. That’s also better and worse than Android.
Better, because it’s kind of a one size fits all approach. They can push an update and the device manufacturers only have to make minor adjustments most of the time.
Worse, because if Microsoft screws up, the whole world is on fire. And you can only modify the OS, not see what it actually contains. Plus, unlike Android, Windows needs heavy modifications for any shape that’s different from the regular. They messed up their Mobile OS platform (RIP WINDOWS PHONE) and while some laptops are experimenting with the form, like the dual screen Yoga Book or whatever, it’s still very much a janky mess.
As a result, Android updates can be largely inconsistent. But that’s not an issue with Android or Open Source Projects, rather the manufacturers who choose not to invest time into making it work. Hope that helps.
Edit: Reworded the comment to reduce the chance of misunderstanding.
There is something off with your reply. GNU/Linux exists and works fine with different kind of hardware while being FOSS.
That’s because GNU/Linux uses open, generic interfaces to communicate with (often fairly generic) hardware.
Android/Linux usually uses specialised closed black-box interfaces to communicate with hardware and those usually only work on one specific device or at best a small family of devices.
This model is dictated by the vendors of the hardware.