Maybe it comes from the fear to gaff out. The more upright you are, the more chances you get to slip.
Perhaps that's less a concern with the very thick barks and the tree gaffs, but with thin bark and/or short gaffs, that's real (at least for me). I'm far from comfortable/confident with the spurs, as I don't trust them at all, so I didn't experiment much. For me, gaffing up 40' is a huge climb.
The way they teach the spur climbing is to keep a good angle between your legs and the trunk. That aims the spikes more toward the trunk's center and put your weight on them within the same axis, instead of downward.
The rule of thumb given in the book is the length of the forearm and the fist, between the trunk and the belly. Legs straight, with the torso in line with the legs (not ass sticking out). You have to adapt it to accommodate the lean of course. That gives some room to work the gear and the chainsaw, reduces the risk of gaffing out, but that's not too far, avoiding being out of reach and keeping reasonable the load on the legs and the back.
At least this spacing works well enough for me. As a bonus, I appreciate that it allows me to have a look down to position my feet.
The legs take a serious beating too by the leverage, especially at the hips.