No matter how accurately they are bent into 90 deg. at the factory, they will get messed up during shipping and handling. So, no matter what, pins will have to be bent back to 90 deg during insertion. And it is easier to always bend pins in one direction than to bend some pins this way and other pins that way.
Let me explain better... Let's say that if a pin is bent 0 deg then really the pin is not bent at all and it sticks straight out and not down. When a pin is bent in shipping/handling it will be bent a max of + or - 10 deg. (I made up that number, but you'll get the idea.) If the pin was originally bent to 90 deg then some pins might be at 80 while others are at 100 deg. So, during insertion some pins would have to be bent IN while others would have to be bent OUT in order to line up with the holes or sockets.
Bending some pins in and others out is difficult to do with a robot. Yes, it can be done, but there are better ways. If the pins were originally bent to 75 deg instead of 90, then after shipping and handing some would be at 65 while others at 85 deg. Then the pick and place machine would only have to bend pins IN-- making the machine much simpler.
To make things better, bending pins IN is easier than bending pins OUT. That's because the same "hand" that grabs the parts only has to squeeze a little harder to bend the pins in. To bend pins out would require a little more work.
I should mention that this point before I get hate mail... The pick and place machines for SMT are frequently differently than TH (through-hole). SMT uses a vacuum system to pick up parts. TH machines could use vacuum for some things, but typically not. Also TH machines often have a device on them called a "lead former". These are normally used to bend and/or cut the leads on resistors, caps, and other devices. I'm sure that there is some machine out there that uses the lead-former to straighten DIP pins rather than using the "hand". But the same issues apply.