If my system is signed and verified at every boot, doesn't that guarantee that my system hasn't been tampered with? Meaning that no malware has found a way to get root access and modify it. I find this valuable.
I do see the value in this and also note that this feature has largely been kept from users intentionally on most other platforms. Still, this offers very little protection for the vast majority of scams to which people fall victim.
If you can't use your own keys and verify the process yourself, then no, that is not a guarantee.
Malware developers just have their software signed by the gatekeepers your device is programmed to inherently trust, because the gatekeepers don't give a shit.
The App Store and Play Store are the largest vectors of malware out there, and every year they are responsible for letting their users get scammed to the tune of billions of dollars.