Why is an input tag not allowed directly within a form tag?

Question

I just read the following at http://w3fools.com/#html_forms:

Non-block-level elements (such as ) are not valid directly insi

Answer 1

It's standards pedantics.

• The statement is legitimate as far as the standard goes: in HTML 4.01, the definition for
  specifies that it may only contain block elements or