Why no 172.5 MTB cranks?

[bcroslin]

I was recently having some fit issues on my mtb so I went to my fitter and he pointed out that since I'm on 172.5 cranks on my road bike (which I spend 85% of my time on) and on 175's on my mtb I could solve part of my fit dilemma by riding 172.5's on both bikes. So I go looking for 172.5 cranks for the mtb and low and behold no one seems to make them - especially Shimano which is what is on my mtb. Any idea why no one seems to manufacture 172.5's for mtb's?

And as a follow-up question, should I consider going to 170's on the mtb? I don't mind a little more spin versus a little less leverage of the 175. My fit issue has to do mainly with saddle height and hamstring issues. I'm on a 29'er so maybe the "spinner" 170 would allow me a little more acceleration?