Last time I had this discussion with a cab driver in SF, he explained that there's some issue with the way cabs are dispatched in SF that pulls drivers to SFO, and that SFO is overserved at the expense of the city. I wish I could remember the details.
Ugh, don't get me started on SF cabbies. They are not only licensed but regulated by price. They also pretend to not know where anything is and will drive around to find your destination, hiking cab fares up accordingly. And, because you may not know the lay of the land (and can't understand their likely intentionally-thick foreign accents), you can't exactly tell them where to go. I had one that claimed to not know the best way to Coit Tower from the Ferry Building, so _he asked me_. Seriously.
It's a big rip-off and I am thankful whenever I visit SF to have a zipcard. Now I only pay BART to get me from SFO to (near) our vacation rental. If ubercab can provide fares without that runaround, I'm all for it.
It wasn't "cabs are lazy".