Upcoming Features in Go 1.18

From here which was linked in the article: https://bitfieldconsulting.com/golang/generics

Why [T comparable] and not [T constraints.Comparable]?

One is inbuilt, the other is just an interface defined in a package? Compiler would check if the types all implement the required operators in both cases, no?

I am no Go programmer and don't know the language specification, just asking out of curiosity.