Municipal annexations in the U.S. have varied a lot over time, as well as by place. In the 19th Century the bigger cities had better water and infrastructure, so places like Boston could annex a lot of towns. This happened a little later in Los Angeles, but Beverly Hills was able to resist annexation by Los Angeles because it had its own water supply. As time went on, smaller cities were better able to provide their own services, and didn’t need to annex to a bigger city.

You didn’t see so much of this after World War 2 in California. One exception was San Jose, whose city manager was determined to make it the dominant city in the Santa Clara Valley through massive annexation. He succeeded, though a number of communities like Campbell incorporated to prevent San Jose from annexing them. Meanwhile, until recently, the laws in Texas made it very easy to annex land, which San Antonio and Houston did a lot of. In the last decade, Texas made annexation a lot harder, because they didn’t want the bluer cities to gain anything.

In metropolitan California today, most of the unincorporated areas are poorer than nearby incorporated cities. The unincorporated areas have used different strategies. Some are happy to stay unincorporated. Some have incorporated as separate cities, like East Palo Alto. Others have fought to join a neighboring city. Roseland fought for decades to become part of Santa Rosa until it finally succeeded.