There are quite a significant amount of reasons why you wouldn't do it, it can also come down to use case as well. Such as how frequently you use the PC and what you do on it.

Due to it being (obviously) the first point at which power is delivered you generally have a good amount of wear on capacitors compared to other bits of hardware in your system. Typically you would want to swap out your power supply every 4 to 5 years, but again it comes down to how frequent you may use your system.

For my own personal reasons, I swap out my PSU after the 4 year mark. It's the one piece of hardware that can potentially kill everything else inside your PC, whereas anything else will either die before it does damage, or it might take out one other thing in the process. It may well be fine for twice as many years, but it also might not, so why risk it?

I mean think about it, that little box is constantly going and feeding the rest of your system with power. For hours at a time, more if you're heavily invested into gaming or anything that requires a lot of rendering, encoding etc.

Depending on what you're upgrading, either a CPU, GPU or full build will also factor in. If it's a simple CPU swap? Sure, it will probably be fine, but once you start swapping or adding more after a few years? It's probably a good idea to at least consider it.

If it's a full new system? Buy a new power supply.

The general consensus is that the one thing you don't bring with you to a new build is the power supply, you also don't "make it last" unlike other parts of your system.

I'm sure someone else can get deeper into the technicalities of why you would want to do this, I wish I could but I would be a little more out of my depth on this subject, as I'm only aware of capacitors and other bits being word down over time.

I believe there's also something else that can occur after a period of time, something to do with power ripple, although again I would be out of my depth on that one.