The transmission gears are engaged by way of gears with dogs (male engagement) and slots (female engagement).
Gear dogs may either slide into their mating gear slots or mesh to their mating gear's dogs depending on which gears are being mated.
The dogs are undercut starting from the outer corner toward their root. This undercut is designed to keep the dogs from slipping out of gear under heavy load.
With a 90 degree angle instead, the sliding gear dogs could simply spin straight out of mesh (pop out) with high centrifugal force.
With the undercut, they can't simply pull out. The sliding gear would have to first separate from dog mesh and then pull out.
The outside edge of the dogs also have a back cut (or chamfer) to help guard against chipping when sliding in or out of mesh against it's mating gear.

¹⁾

The gear dogs can wear on the sides. ²⁾ They will sometimes become rounded off instead of being flat on the edges. This can cause the gears to not sync 100% and pop out of gear.

Worn dogs 3)