CPUs do have bugs, but that doesn’t mean that’s the explanation.
As someone who worked on an OS release in the past, I’m going to guess that this is more about testing resources. Microsoft are probably pretty sure Windows 11 works just fine on older CPUs, but the thing is... if it doesn’t and one of their major enterprise clients sues them over it, they need to show that they made an effort to test it before release.
You cannot test everything, and there are now three platforms to deal with (ARM, Intel, AMD), each with their own quirks. Windows 11 is a free upgrade for older users, so there’s no revenue to offset that cost (for new systems, the motherboard vendors and PC makers do a lot of the testing). That limits how broad the testing can be.
Unlike Apple (where I was), Microsoft can’t just open up a spreadsheet and see every system configuration their old OS is running on: Windows is expected to work with every possible configuration of each supported chipset, including some that Microsoft’s engineers will never have seen. It’s a credit to the industry’s efforts at standardisation that any of this stuff works so well at all, but there will always be some oddball system that trips up (and that goes for Linux too).
Windows 11 has considerable changes to its kernel, much more than any release since Windows 7. Those changes are needed to support the next generation of CPUs with a mix of high- and low-performance (=low power) cores, but some may have negatively affected performance on older systems.
I do expect that as time goes on, those issues will be resolved for popular older CPUs, but right now, the priority is on the people who would actually buy the OS: new PC owners, and people who are likely to buy a new PC.