Why is Microsoft Windows so often unstable?
Windows is NOT typically unstable (in quite some time) UNLESS it is loaded with buggy drivers or services (usually not by Microsoft.)
Not that Microsoft is not necessarily any better at producing software but because they are more likely to test their own OS more fully, and because so many people use Microsoft products and tend to find most of the major incompatibilities very quickly.
My Windows systems generally run without trouble, run quickly, and only get rebooted to add updates. Were it not for updates, they would likely NEVER get rebooted unless a power outage went longer than their battery backups. :)
If your Windows machine is crashing, it has problems.
I have been using my PC for over 3 years, it has never blue screened on me.
If this question is genuine, and you're asking why Windows is unstable. The answer is that it's not, and your PC has problems.
We're not resigned to working with systems that crash often, all the modern platforms in 2017 are pretty rock solid.
If your machine is crashing, start another question and post details.
It is almost certainly a driver issue related to network drivers, video drivers or other peripherals for which you've installed third party drivers. If the drivers are signed and vetted by the Microsoft Hardware Lab then I'll bet they won't crash. If they downloaded and installed from the manufacturers site before they undergo WHQL certification, they may not have been adequately tested.
Bottom line: it's probably not ‘Windows', although I'm sure that is small comfort.
The problem is that Microsoft Windows is an open ecosystem. This means that it's third party vendors that make most of the devices (and the device drivers), not Microsoft. Microsoft Windows must run on any piece of hardware, and it's the hardware designer's responsibility to make sure it runs bug-free. Frequently, the hardware drivers are not bug-free.