They have been working on the 10nm manufacturing process for ages and it's still not completely finished. That gave AMD huge opportunity to catch up (which it did).
IIRC most of Intel's expenses are related to the fabs and semiconductor R&D. The chip designs (the computer architecture part that gets computer people excited) is almost a rounding error on top of that. From that perspective Intel designs chips so that they can sell the silicon real estate they manufacture, not the other way around (that they have fabs so they can realize their chip design ambitions).
So if they would fab their chips somewhere else, they would be sitting on a huge expensive asset producing nothing. If they couldn't find a productive use for their fabs it would likely mean the end of the company. And if they can't produce their own chips in their own fabs, why would anyone else want to use Intel fabs?
Further, the vertical integration of fab process and chip design is something Intel regards as a competitive advantage. For a long time this was very much true, but it seems the hard work by TSMC and others have made it possible to make top-end chip designs on a merchant foundry process nowadays.
Intel at some point tried to play the merchant foundry game, but it seems it wasn't successful and they shut it down. Which perhaps isn't that unsurprising, considering TSMC, and to a lesser extent Globalfoundries, have been at that game for decades and they're good at it.
So all in all, I don't think fabbing their chips at some third party is a viable approach for Intel. Either they fix their process or they go under. "Go under" not necessarily meaning bankruptcy, it could also mean a massively, hugely downsized company doing chip designs to be fabbed at some third party. I think they're still far away from such a drastic step.
I'm not a hardware engineer/cpu designer/electrical engineer, but my understanding is that designing high end cpus requires engineering and designing towards the specific manufacturing process of the fabs you're using. Even if intel did decide to, I don't think they could just send their designs to tsmc/global foundries or what have you.
Not my expertise either, but it seems that the advantages of vertical integrations between fabbing and chip design being large was the "common wisdom" in the industry for a long time. But it seems that lately AMD/NVIDIA/TSMC and others have demonstrated that fabless chip companies and merchant foundries is a model capable of producing the highest end chips as well.
And yes, if for some reason Intel would want to fab their chips at some merchant foundry (see my sibling answer to yours why I think that's unlikely, but just for the sake of argument), I'm quite sure they couldn't just email the RTL's to the foundry and get chips back. It would take a lot of work to adapt the chips to the merchant foundry's process.
They cant, they will need to redesign the whole thing with new rules, mask and testing. Not to mention using tools that likely Intel is extremely unfamiliar with. ( At least their CPU teams.)
It is more of a cost/ economical issues, which jabl above provides a decent explanation.
Either it isn't worth it (too much coupling between fab and designs - not the right abstractions), or institutional inertia stops them doing it.