If you pose that question to Larry Brock. Field Engineer of FCC's Dallas Office, he would likely tell you the reason is one word. Interference. It is why the rule was initiated to start with--along with that bugaboo about CB as a hobby is against the law.
Sorry Jerry I believe you might be flat wrong about that. A couple years ago I did a little research on where the 155 mile rule limit on CB came from and why it was created. The best answers I found in my research is as follows:
During World War II, first all international ham communications was prohibited, then later all amateur activities except those specifically sanctioned by the military became prohibited. Why was that? Because of espionage concerns.
The cold war started & sanctions were lifted. At the time CB and Ham shared the 11m spectrum and CB required a license. However, there were already certain other HF frequencies where unlicensed usage was permitted, dating back to 1938. At that time, power restrictions had been put in place to keep the communications at a short distance to avoid crossing state lines. At the time, congress only had limited ability to regulate commerce, and stuff that crossed state lines fell within certain existing laws that would preclude the feds (FCC, in this case) from allowing unlicensed operation.
When what was the precurser to the modern CB service was created in 1945, the 155 mile rule was put into effect for two reasons: Because of concerns on the legality (at the time) of FCC's authority over anything that crossed state lines, and also continued concerns over espionage.
In 1959, Donald Stoner published a detailed article on how to design and construct a home made CB. At this time, it was not illegal to do so, and commercially available kits became available. By definition, CB at it's earliest stages was experimental. It wasn't until much later that the FCC decided to try and change that.
In 1977, when CB expanded from 23 to 40 channels, there was actually talk about expanding all the way to 27.995, but it was decided against to prevent intermod breakthrough to any 455kHz receiver that used the 455kHz IF stage.
Are the same concerns that started the 155 mile limit still concerns today? I doubt it. Maybe that's why the FCC doesn't enforce it. Remember why the FCC dropped the CB license rule? Becuase everyone ignored it, and they decided it didn't matter anymore since they already had a protocol all the way back to 1938 that could allow it.
Definition of hobby: "An activity or interest pursued outside one's regular occupation and engaged in primarily for pleasure." (source dictionary.com) The government can't regulate one's intellectual interests, and according to existing rules, it is perfectly ok to talk on the CB "primarily for pleasure". So by definition of the word hobby, the FCC cannot regulate CB as "not a hobby".
BTW, another analogy keeps getting used: speeding. Does anyone know or remember why the 55 mph national speed limit was created? It was created by Nixon as a result of fuel economy concerns during the first oil embargo of 1974, and that was the ONLY reason. It was an arbitrary limit that was set that incidentaly gave rise to mobile CB use to look out for cops while ignoring what many felt was a stupid law.