stupid question?

Yes, ideally you'd have 3 of the exact same speakers for L, C, and R. However, center speakers look the way they do for convenience. It's easier to place a center speaker on a TV or shelf above or below the TV when they are configured MTM horizontally. Some manufacturers will have L, C, and R speakers with the same drivers, but the center will still have them arranged horizontally, which can sound different from a speaker with the same drivers in a vertical arrangement. However, getting matching speakers across the front is a good compromise even if the center is configured differently.

To add to what deneb said, another practical reason that center channel speakers sell is that it's usually hard to buy a single bookshelf or tower speaker for the center, as many speakers are sold in pairs only. So, in addition to not having the space for a vertically-oriented speaker, people are sort of "forced" to buy center channel speakers for the center.

The reason horizontal centers are "bad" is that horizontal speakers don't disperse the sound properly in the horizontal plane, which is where you'd most like your center to disperse sound, so that everyone in the room can hear it well. Horizontal speakers disperse sound best in the vertical plane, which is exactly where you don't need your center speaker's sound to be dispersed; to the floor and ceiling. There is a reason why almost all bookshelf and tower speakers are vertically-oriented; they disperse sound best in the horizontal plane with a vertical orientation, which is exactly where you want them to disperse sound.

If you can get a third speaker for the center that identically matches your front L+R speaker, and you have the space to orient it vertically, then that's what you should do.