Not to short-circuit the joke, but in this case, it's because the valid JavaScript version is...

let a

...and one of TypeScript's main design goals is to be a superset of JavaScript, that only adds syntax, and doesn't re-write it.

Beyond that, it's probably a case of some new language just using what the designer is familiar with.

I've always wondered where all this 'let' business started

Can we talk about PHP functions with typehints too?

public static function foo(): string {

Practically every other language with similar syntax does this instead:

public static string foo() {

Javascript gonna Javascript