Because technology is ever-changing and improves.
Don't do the upgrade. When I bought my ASUS desktop, it had Windows 8.1 on it. I simply downloaded Windows 10 (because I was eligible for the free upgrade) and I did a total clean install instead.
You want peak computer performance, then you need to update to the latest program.
I have done many upgrades and fresh OS installs and it is not a trivial thing. Follows if it isn’t broken don’t fix it rule.
Also why most businesses still run win7. Btw, we bought a new PC and had Win10 installed on it. Win 7 was an option. Seems contradictory but it isn’t. It makes sense for new systems.