Why do lathe spindles use coarse threads?

[holmqer]

The only problems I've seen with lathe spindles is being bent, or the threads not matching a particular chuck or faceplate. The later is probably a tolerance thing since every screw and nut has a tolerance and if you happen to get a spindle on the plus side and chuck on the negative side then problems arise.
The bent spindle seems to be more common. I have not seen it yet on 1 1/4, m33 or 1 1/2 but have seen it several times on 1" spindles. Usually from putting too large a piece of wood on a lathe.

The minor diameter of a 1-8 spindle is 7/8", the MT2 is 3/4" at its widest, so the wall thickness on a 1-8 is 1/16" at its thinnest, with a metallic cross section of 0.184 in^2.

The minor diameter of a 1 1/4-8 spindle is 1 1/8", so the wall thickness on a 1 1/4-8 is 3/16" at its thinnest, with a metallic cross section of 0.625 in^2.

Thus, a 1 1/4-8 is around 3x stronger than a 1-8, while a 1 1/2-8 would be around 6x stronger than a 1-8. The better bearings of the larger lathe probably also helps.