We already do this. Consider Las Vegas... its water comes from the Colorado River. (Edit: to be clear, the Colorado River itself is transporting water from areas with high rainfall to areas with low rainfall. The Las Vegas area is a desert, but most of the Colorado River basin is not.)

It's just not generally done over long distances. Oil is pumped over long distances, but it's also much more valuable. For water, we rely more on cheaper ways of transporting it, and transporting it over shorter distances, using things like aqueducts and, yes, pipelines.

Here's an example of a big water pipeline project:

https://en.wikipedia.org/wiki/Northern_Cyprus_Water_Supply_P...

Just adding to this, not far south of Las Vegas the Central Arizona Project is a rather large canal (so not exactly a pipeline but close enough) which transports Colorado River water to Phoenix and Tucson.

https://en.wikipedia.org/wiki/Central_Arizona_Project

We already do this. In Colorado there are over 50 "Transbasin Diversions" that move water from one river basin to another. Most of them transfer water out of the Colorado River Basin to the east side of the continental divide, usually the Arkansas River or South Platte River Basins. California also does this on a vast scale to move water from the wetter north to the drier south parts of that state.

There are many issues with transporting large quantities of water vast distances. From legal (it is illegal to transport water from the Great Lakes to outside of their basin), to physics (water is expensive to pump up and over mountain ranges and building tunnels is incredibly expensive.

Basically, just like there are only so many places that it is cost effective to build dams or pumped hydro electricity storage, there are only so many places that it is cost effective to build pipelines.

We do this in some cases, but in general oil is expensive and water is cheap. A gallon of crude oil costs around $2.50. A gallon of tap water costs a little over a penny where I live (in San Francisco). Transporting expensive things is an easier sell than transporting cheap things.

Plenty of long water supply tunnels around too[1].

[1] https://en.wikipedia.org/wiki/List_of_longest_tunnels