system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do computers get slower and unresponsive the longer they are left on/over general time?

My computer which I built a few years ago isn't old enough or obsolete enough yet to tell me it can't reach any options on ctrl-alt-del. So why did I just essentially get forced to restart my computer? Why do I have to deal with perpetual slowness and waiting for the computer to "catch up"?

I run a very tight ship so if it is a problem on my end it isn't because there were hot singles in my area.
gpt
Programs and drivers can "leak". This means a program asks for a little memory to use and doesn't give it all back when it's done.  If there's a section in the program that's called over and over, it will slowly use up all your available memory. When your computer runs low on memory (because it's all allocated to this wacko program) it starts using disk as memory which is really really slow, making things worse. 

Rebooting causes the computer to clear the memory and start fresh where the whole thing can happen again over time.

It's incredibly difficult to track down leaks as the OS will not report what's using the memory accurately.  Newer versions of windows are better about dealing with this than versions from a few years ago and horrible design choices aside, this is the kind of stability they're working toward.