Ahead-of-time type checking can only really work if the entire program has type declarations everywhere, and if you know what constitutes the entire program. This isn't the case for the vast majority of existing PHP code. Furthermore, PHP's dynamic features mean that there are cases where ahead-of-time checks will always be impossible. So you have to have at least some runtime checks, at the boundaries between typed and untyped code.

Furthermore, doing ahead-of-time checking would require infrastructure we don't currently have for providing information to the PHP interpreter about whether or not PHP files have passed type checks. But again, most code is not fully typed, and so this doesn't provide all that many guarantees to the interpreter.