What you need to understand is that we are talking about SHEAR.

"Shear" is when two objects (in this case, the spars) move in opposite directions to each other.

As air pressure lifts the wing, and since the weight of the fuselage is at the root, the wing tries to lift at the tips. This causes the spars to try to move in a shearing motion. (First image)

Although the pressure is at 45 degrees, the movement of the spars is relatively 180 degrees from each other. So a horizontal grain is more likely to split than a vertical grain.

You COULD add strips at 45 degree angles, but this will result in less gluing area. Of course, you could also cut full webs with a 45 degree grain, but for the most part, this is overkill.

The bottom line here is: I have built MANY planes with a vertical grain web and have absolutely tortured them without a single wing failure.