Because you have a phone with iOS. iOS does not permit you to download software. You can download apps, if there is one in the App Store, but there isn't an App for Xcode 12 in the App Store for iPhone.
Xcode 12 is built as a Universal app that runs 100% natively on Intel-based CPUs and Apple Silicon for great performance and a snappy interface.* It also includes a unified macOS SDK that includes all the frameworks, compilers, debuggers, and other tools you need to build apps that run natively on Apple Silicon and the Intel x86_64 CPU.
As you can see, this means Xcode 12 will run on an intel based Computer and on a Mac which is Apple Silicon based. But it won't run on an iPhone.