Because it's undeclared.

First one needs to declare a variable to define something.

a := 5     // declared and defined
b := a + 5 // RHS is evaluated, then `b` is declared and defined, `b` can be used from next line
c := c + 5 // RHS is evaluated, undeclared variable `c`, and `c` is undefined

A variable is declared only at the end of it's expression inside a function.
i.e., f() is declared and defined only after whole expression of f() is evaluated, (i.e., after RHS evaluation is completed).

See Point 5: https://golang.org/ref/spec#Declarations_and_scope

If you try the same in package block, you will get into invalid initialization cycle.
https://golang.org/ref/spec#Package_initialization

var a = a + 5 // invalid initialization cycle

https://pkg.go.dev/golang.org/x/tools/internal/typesinternal#InvalidInitCycle

This one is valid since it's declared and then defined.

var f func()       // f is declared
f = func() { f() } // f is defined

Closures are different. Assigning an undeclared variable (c = c + 5) doesn't work in Python and Javascript (c is not defined), but these do:

Python:

f = lambda: f()

Javascript:

let f = function() { f() }

By the time the Go compiler gets to the function body, it already knows the type of the variable f.

Duplicate of #33167