The general public tend to equate number of speakers with quality of system. Hence, the upper trim levels of many cars have "818 speaker sound system!!!" marketing.
One of the most beautiful sounding systems we ever built had two tweeters, two mids, and one sub. But, the system was built with insanely high quality components and countless hours of imaging analysis to position the speakers, tune the crossovers, and so on to create a soundstage. When you attend a concert, none of the band sits behind you. So, you don't really want any frequencies above the bass to be projecting from the rear.
Even on many luxury cars, we found crappy paper cone speakers that would have been cheap in a base Accent, much less in a $50K car. We did many fairly inexpensive upgrades by adding a small line-level amp (many factory head units don't have pre-outs) and better speakers. We typically only added power to the front speakers.
The best factory system I've heard is the Mark Levinson Lexus system. A lot of effort combined with superb components really make upgrading those systems either difficult, or a case of "why bother?"
Don't get me started about Bose...