Ranchers built their ranches with what was available to them, and with what was possible given their time and personal circumstances. They could vary greatly depending on when and where they were constructed. One reason, therefore, would be resources - in much of the Southwest and Plains, trees available and useful for construction are scarce, and adobe bricks are time-consuming to make and also require another scarce resource - water.

Another reason would be manpower - a watchtower is pretty useless unless someone is actually keeping watch more or less all the time, and on a single-family operation, that just might not be feasible. Even a small spread would have times when all hands were needed outside of the ranch itself. It works in a village, where you have a large pool of people to staff it, but not really on a homesteader's 160 acres.

I would also argue that most settlers of the era did not personally emigrate from Europe. While there were certainly no small number who did, most Euro-Americans would have no personal (or cultural, given the development of American cities) memory or preference for a tower. Futhermore, again strongly depending on place and time, defense from whom? In certain places and at certain times, the threat of a Native American raid was very real, but much of the "Wild West" was really quite peaceful - gunfights in the street were very rare, and there were certainly no city-states with marauding armies threatening your community like in medieval Europe. Rustlers might attempt to steal your herd, but given the terrain and size of some ranches (especially in the Southwest) even the added height of a tower wouldn't help a rancher see all of his range, or enough to find it useful.

Sources: Elliot West, The Contested Plains; Richard Fracaviglia, Hard Places; Rhyolite Historic Site; Lincoln Historic Site.