Cycling is interesting. You can get a frame in nearly any size in one cm increments from 49gm to 63cm. You can get cycling shoes in a 37.5 up to 52, with weird 1/3 and 2/3 sizes in some French shoes. You can get a stem with various degrees of ascension and declension and in all manner of extensions from trick stems to stems over 150mm long. There are 650, 700c, 26", and now 29er wheel sizes, plus a host of others that we typically don't see here in the US. Seatposts come with varying degrees of setback, to zero setback posts, even to offset posts for time trial use. You can get STI levers in smaller sizes for people with little hands. Handlebars come in a variety of widths, reaches, and drops. Pedals come in different amounts of float from completely biomechanically neutral pedals like the Bebop, to old Look style that convey proper set up or a visit to your Orthopaedist.

However, while nearly everything can be 'sized' in cyclesport crank length is often considered 'universal' for cyclists that are as divergent from 5'7" to 6'3".

Having cranks that are too short or too long can severely affect your spin. Truly, the average cyclist has never been professionally fit on a sizercycle by a certified fitness specialist. They have never tried different crank lengths to determine what is appropriate for their cadence, nor do they necessarily choose their rings and cassettes to accommodate their cadence, and to provide the broadest usable range.

Many cyclists are not adequately positioned over the pedal axle during the pedal stroke.

I may not be that you "can't" spin, but rather that your bike isn' adjusted, or does not fit, thus preventing you from spinning. It is cheap to manufacture cranks in only 175mm.

However, its absurd to simultaneously believe that we need 16 different frame sizes, but largely only one crank size.

Your ability to spin or mash will be completely different on different length cranks. I always found it curious why people would spend thousands on a bike but never think to spend a couple of hundred on a fitting tool like an Ergostem, that they can use to make every bike more comfortable, and improve their performance upon. The same goes for crank length, I'd encourage any serious cyclist to invest in an adjustable crank. Be sure to find one that has an adjustable range proportional to your size. Having an 160mm-180mm adjustable crank isn't going to help someone 6'8" to determine what the ideal size they might need.

Don't assume that a particular cadence is ideal for everyone. There are different natural body types, and different natural cadences. The battle between Jan Ulrich and Lance Armstrong on time trials emphasized the efficacy of mashing and spinning. Both were world class time trialists almost without peer. Their respective cadences could not be more different. It has certainly been indoctrinated into the current generation of cyclists that they need to 'spin' at a higher cadence.

While its currently de rigueur to spin at a higher cadence the aforementioned Ulrich, Santiago Botero, and Big Mig all were very fast at much lower cadences.

Not every cadence is appropriate for a 'universal' 175mm crank, however.

Crank length and natural style are oft overlooked variables in finding your own 'groove'. There is not one pure spin, and what works for someone five foot nothing and someone who can dunk flat footed is not, by any stretch of the imagination, the same.

Vastly overlooked, and what you have begun to discover intuitively, is that a higher cadence actually leads to a lower max wattage, as the muscles don't have time to adequately recover between exertions, and that lactic acid will build up quicker than it would otherwise.

There is not one 'best' way, regardless of conventional wisdom.

One could make the argument that had Ulrich had equal access to cutting edge pharmacology we'd all be talking about mashing big gears to go faster now...