Because
1. golf courses take a lot of land and maintenance to maintain. Lets assume you don't care about the grass part because hey its a wasteland. You still would have to have a huge amount of land that you will need to fence off and protect against any ne'er-do-wells. (ie: raiders and robbers) Furthermore, you will have to hire people to take care of and protect all this land and your customers. Even a 9 hole course is too much to guard for 1 person. Plus, putting is more complicated than you would think. You need SMOOTH or FLAT ground for a ball to be putted. You can't just put a ball with tons of divots, dips, potholes, and other uneven and rocky grounds that would be in the wasteland. That's why putting greens are so well maintained and built. And this is all assuming that you CAN get your hands on some pre-war clubs and balls or even manufacture new ones (which would svck).
Even if you had all of that taken care of.. Lets say you had barbed fences with auto turrets guarding the entire perimeter, you had a staff, and kept the land maintained, and you had plenty of balls and clubs, why would anyone come and pay you for this? Most people are trying to SURVIVE, even if they did have time for such luxury, all of those resources (manpower, equipment, autoturrets, fences) are better spent on rebuilding society.
Baseball is an easy and cheap sport to make. You need a field, a ball, and a bat. You can make bases out of anything and even catch the ball with your hands if you needed to. (wouldn't recommend it though) But another reason baseball is mentioned more is that it is America's favorite past time. It fits the 50's theme quite nicely even though golf was around at that time as well
2. If you mean why weren't they mentioned at all. Camp Golf in New Vegas was a golf course.