How to get outlets straight inside boxes?
I would hazard a guess that the boxes are not aligned perfectly. This isn't uncommon, and often the metal flanges of the outlet will need to be slightly, deliberately twisted to get the face of the outlet to match the plane of the plate (which will pretty much always conform to the wall). Another factor, as mentioned by @Tyson, is the force of the wires behind the outlet pushing it out of alignment. This can particularly be a problem with 12 guage wire and bulky GFCI outlets.

In extreme cases of vertical box misalignment, you may need to not clamp the outlet flange tight against the box (either leaving it slightly floating, or shim with washers) but in this case it looks like most of the misalignment is horizontal, and can be remedied with a little twisting (but you could maybe back out that top screw a turn or so).

It's also possible to snap off the "ears" on the flanges if any of them are impeding alignment, but it's hard to tell if this is the case from the pictures. Alternately you may need to gouge out a little more plaster under the ears on one side.