It's not a result of anything Max related. Modelers who use other programs also use that pose - and actually, many tutorials on modeling humanoids in Max have you modeling the classic T.  Most of the figures I make are modeled in the 45 degree pose (which is why if you Zero Rex and Roxie the legs are shoulder width apart for both) but because of the reasons mentioned above, their arms get placed in the classic T pose.  Then, because IK is just a fun little experience, we have to find an ideal default leg pose that will work with IK without causing the legs to buckle in a painful way. Sometimes, despite having the legs in a position that would appear to be ideal, we need to adjust the default pose in less than natural looking ways to get the IK working.  There's a lot to consider when modeling and rigging for Poser. People don't really get that because many of you don't do it but believe me, everything has a purpose.

 

That said, as I mentioned, there's nothing stopping someone from setting up a figure in the 45 degree pose. There will be downsides to this but that's the case with any default pose. Just the nature of the beast really.