Depending on the mission it may not be possible to get back to the landing spot and sometimes the most efficient place to land is in the middle of the ocean. Musk answers that question here[1] in a tweet regarding the launch. You would have to have launch sites all around the earth and get approval to land or carry a lot of extra fuel to get back to where you started from which would be prohibitively expensive and heavy.
Nobody else does this. I have absolutely no idea what you could possibly be referring to. SpaceX lands on land preferably when it is an option.
> I don't understand why they won't keep landing them on the ground ... It's not like there's not enough space on Earth
As a general rule, you not understanding the reason for something does not mean there is not a good reason. For reference, see the Dunning-Kruger effect. Furthermore, I honestly do not understand how you could even contemplate "SpaceX does not think there is enough space on land" as a serious argument and treat its dismissal as legitimate support for your beliefs. This is some of the most egregiously lazy reasoning I have ever seen on the internet.
SpaceX launches rockets in the direction of water so that an accident won't threaten property or people on land. To return to land after staging the first stage of the Falcon 9 must perform a burn—the "boost back"—to point its lateral velocity back to land; this is the first of three burns required for landing and the most expensive in fuel by a considerable margin. If the first stage is travelling too quickly (greater than approximately 1.6 km/s lateral velocity) it does not physically have enough fuel to return to the launch site and perform the landing. In this case, a far more mild burn can put the booster on a re-entry trajectory and it can land on an autonomous barge in the ocean. As the rocket trajectory is chosen to put the ground path well away from land, the choice is between either returning to the launch site or landing in the ocean. If a return to the launch site is prohibited by the rocket's capabilites and physics then an ocean landing is the only option.
In the case of Jason-3 a barge landing was the only option as SpaceX does not yet have permission to use their landing facility at Vandenberg Air Force Base. In the future, the central core of the Falcon Heavy will almost certainly have to land in the ocean due to its velocity at separation.
The Space Shuttle was a space craft, not a rocket. The SpaceX equivalent to the Space Shuttle is the Dragon spacecraft. Dragon V2 does land exclusively on the ground.
OK, that's one orbital rocket that's recovered on land. You said "everybody else" which implies at least several, so what other examples are there? (Hint: none.)
The space shuttle couldn't land and take off on a planet whose atmosphere doesn't support it, hence why SpaceX is trying to do the same via rocket propulsion.
Not to change your overall point, but you almost never launch west. When possible, you want to launch east, to gain the benefit of the Earth's rotational speed. Sometimes you launch north or south so you can go into a polar orbit which allows viewing more of the Earth. But there's pretty much no reason to launch west. One exception to this is if your launch site is located in a place where that's the only option, which is why Israel does it like this.
For American launch sites, you have KSC for launching east, and Vandenberg for launching south. (North and south are equivalent in terms of which orbits they can reach, so you don't need both.) Vandenberg can be used for launching west, but pretty much never is, unless you count ICBM tests and the like.
I assume the parent poster meant that the launch should take place from somewhere on earth where there is earth 2300 miles out. It's likely not convenient to do so, at least not for all launch directions.
It would be convenient enough but the FAA isn't too happy with the idea of a 12-story missile full of explosive fuel and oxidiser flying over populated areas.
If you just needed to find two bits of ground with room for rocket pads and one 2300 miles east of the other it really wouldn't be that hard to have both in the continental US. It's the fact that the whole flight path has to be safe that makes it virtually impossible.
[1] https://twitter.com/elonmusk/status/688844190826942465