That's basically my experience for 2 other "gaming" motherboard brands that aren't ASUS as well. My guess is that people who build their own PCs probably don't care about SMBIOS serial numbers being properly populated, so why bother?
I would care if I could change it, but you need a proprietary tool that you can't obtain. (Every other way I found involved patching the UEFI and turning off Secure Boot)
