As for the no walkaround during turns, I think I remember in training that our rampers get training so that if they see something wrong (bird hanging from a wing leading edge) they let the pilots know.
For pulling into a gate, I suspect that we don't need wing walkers because of our one plane type operation. All of our gates have red and yellow lines painted by our company that are exactly the right size for our planes. The pilots clear the left and right side visually (and verbalize, "safety zone clear right.") I could see that if you put 767s and 737s and airbus types into the same gate, it could be much more difficult. We've all seen the multiple stripes painted for the different aircraft that use a gate.
That is my guess at least, just another fortuitous result of operating one plane type. (Granted, the 700s with their winglets are a little wider so there are a few gates in the system that are not for 700s but most gates fit either 300 or 700)