Engineering inertia is certainly a possibility.
Early typewriters had a round metal frame holding down a clear piece of plastic (or perhaps mica!) over a round piece of cardboard with the character printed on it.
So, although I remember some Underwood office manual typewriters having a cylindrical key surface, generally molded plastic typewriter keys had a spherical indentation. Partly to preserve the feel of keys as they existed before, and partly so that touch-typists could locate keys in two dimensions. (Deeper indentations, or bumps, on the F and J keys are a computer terminal thing, not a typewriter thing.)
Early IBM electric typewriters (less early than the one whose photo I recently posted) - and the Flexowriter based on them - had Bakelite keys that were 1/2 inch square, with a circular area containing a spherical indentation. (Because they were Bakelite, they couldn't be double-shot molded, but instead had the white letters filled in.)
Later IBM electric typewriters, the Model 29 keypunch, and the Selectric, of course, got fancier. But the keys were spherical because that's what everyone was used to.
Then the PC came along, allowing transparent overlays on the keytops by having a cylindrical surface, and suddenly cylindrical keys with left-justified characters became "modern" and fashionable - IBM started the trend, and everyone followed.
It's not unlike see-through calculators coming out soon after the iMac.
There may have been no particularly good reason for the old way, but there wasn't much of one for the new way either.