If not, why not? I'm not advocating for it, just wondering what the argument against it is?

WN, B6, and VX don't count??

You don't have to fully agree with it, but here's an argument:
Part of FF programs is to entice you to fly another carrier even when you have other options (perhaps even cheaper options). Generally, the further you're flying, the more options you have. The shorter the distance you're flying, the fewer options you have, and thus the airline has to do less to entice you to fly them. If you're spending $15K a year and only flying enough to get FO, yet are still giving DL $15K in spend each year, what incentive does DL have to entice you to fly them more? You're clearly flying DL because you have to or don't really have any other viable options (lack of OAL competition). Ie, your purchase decision isn't being driven by FF benefits. OTOH, if you are flying LA-NYC, you have plenty of options and the FF benefits entice you to steer your business (and thus $$$) towards DL.

Again, you don't have to agree it. I'm not saying it's perfect logic or that I even buy it, but I have seen that argument and in some regards it does make sense.

Probably not. The most likely reason is because spend alone doesn't directly correlate to profit, only to revenue.

Airline tickets prices and operating costs have so many factors that it's nearly impossible for anyone to fully analyze but in a simple sense, flying shorter distances can actually be more valuable. An $800 ATL-DCA last minute fare is more profitable than an $800 ai fare from ATL to Europe or SE Asia. However, one trip doesn't even net you 1100 MQMs; the other trip nets you nearly enough MQMs for FO status.