why are some toilet seats not round, but horseshoe shaped?

The horseshoe variety are used in public ammenities because of the drip factor , so the next guy who comes along doesn't have to wipe the piss off the seat before he takes a seat.

There are two different styles of toilets, the round and the elongated ones. (A few years ago, there was a third one by American Standard - but this is no longer produced.) The standardization of either the round or elongated toilets allowed for easy replacements for parts - such as toilet seats from your local hardware store. The difference between round and elongated is pure preference. As a guy, I would prefer an elongated one. The gap in the front of the public toilets is a sanitary issue. Most guys are slobs in the public restrooms and don't pick up the seat. The gap is supposed to help prevent a mess. (Doesn't work.)

The first reason is because the part of the seat that usually fills the gap sometimes gets in the way of Junior. The other is that it costs less if the toilet seats use less material.

The toilet seat you are referring to is called a male toilet seat. It was designed that way so that it would be more comfortable for men to urinate while sitting on toilet defecating.

So you can spread your cheeks and take a glorious dump.