Those products do have them, but with significant differences of form. The Note 3 is 10% thicker than the iPad Air, and yet the digitiser has to support only a bit more than half the surface area. Likewise the SP3 digitiser does support a greater surface area than the iPad, yet the Surface is 20% thicker than the iPad and has 75% more weight. As you agreed later in your reply, the iPad is, first and foremost, a consumption device, or at the very least that is Apple's philosophy regarding it, and part of its evolution has been to decrease its footprint with each successive generation; indeed the near-universal acclaim for the Air has been its "impossibly thin" form factor. Since Apple obviously won't take a step back in that regard, nor should they, I stand by my original statement that the engineering challenges to incorporating an active digitiser into the iPad without sacrificing that factor would be an enormous undertaking.
I'm certain that there are conceivably other companies out there, but I question how many, if any, have the resources or competence to produce a product that can compete with Wacom. Both N-Trig and Samsung's digitisers are acceptable for note-taking, but for line art they're a joke. Only the Cintiq, a far more expensive device than the iPad, even reaches passable performance.