Morgans wrote:

The higher HP is a major reason, since you rarely see flat engines with HP
ratings of more than a bit over 300 HP. There are many radial engines that are
much larger than that.

Lycoming built a 12-cylinder opposed engine. I don't think it ever
went into production. There's a picture of it he
http://www.prime-mover.org/Engines/Images/lyco1230.jpg

The home page for a bunch of interesting engines, including Lycoming
auto engines:

http://www.prime-mover.org/Engines/Pictures.html

Dan