I'm not an expert at parsing, but one main issue I can imagine with having the same syntax for declaring functions and variables is that you need to look at six tokens to understand if it's a function declaration or a variable declaration. I say six because you have the name, colon, type, equal, parentheses (certainly you can't use this because then I couldn't declare a variable using parentheses, e.g., foo: int = (4 + 5) * 3;), and finally a pair of curly braces. And even then, I don't know Jai at all so it may be legal to use them in variable declarations.

With the name := () -> return_type {} syntax, you know whether it's a variable declaration or function declaration after two tokens: name (for either), then colon or colon-equal to differentiate the two.