The problem with this canard, or at least misconception, is that it takes the notion "I would buy this kind of thing if Apple offered it" (which may or may not be true) and assumes that, therefore, it would be a good business move for Apple to offer that configuration.
In the mid 90s, Apple had so many product lines and options that you couldn't keep track of them: Classics, Performas, Quadras, whatever. There were Apple-manufactured machines that had two processors for dual-booting, not to mention several brands of clones. (This is another thing that many people still say: "if only Apple would get their head out of their asses and license their OS to other manufacturers, they would increase their market share, blah blah blah...") At that time, it looked like Apple wasn't long for this world.
After Jobs came back in 1996 (1997? whatever.), the company slowly reined in the product lines and started to concentrate on making a few identifiable, distinct products, with a limited number of options for each. Apple is now a quite successful company, and, while their non-computer products are a large part of this, the company has managed to continue to hang on to, and even expand, its corner of the computer and OS market, a market that is surely stacked against it. Not only that, Apple has become a trendsetter in this market.
You can bet that there are some pretty savvy financial analysts at Apple who have probably looked at this a lot more closely than you have, and, if they really thought demand was high enough for a mid-range tower, they would make one. I would bet that the average computer user (not the average Slashdot reader, which is something else) never expands their PC past the basic configuration that they bought it with during its lifespan, and, furthermore, doesn't need anything more powerful than what comes with a Mac Mini. The population of customers who need more than a Mac Mini, but less than a Mac Pro (like you) is real, but too small to be profitable for Apple. Apple's success is not based on a shotgun approach but on carefully maximizing the profitability of a small number of product lines.