Have you bought a PC in the last 10 years? Then it came with Microsoft's secure boot keys on it. Sometimes it's not even possible to remove or disable them. Sometimes you actually need a Microsoft-signed bootloader shim to boot anything that isn't Microsoft.

I haven't bought a Windows PC in the last 10 years, yes. I think the last Windows PC I bought was a used ThinkPad from 2011 that I upgraded and used until 2022.