I made a map with Great Circle routes from a few major US terminals into SFO. Please refer to:
As you can see, pretty much anything inbound to SFO from south of ATL (MIA, DFW, IAH, and PHX, in the example) have shorest distances that take them over LAS. Other stuff from points north of ATL (IAD, JFK, ORD and MSP, in the example) takes them well north of LAS, and south of RNO.
All that said/shown, the shortest distances aren't always the ones flown, obviously. ATC has a big hand in this, as was alluded to, as they try to design the traffic flows so as to avoid saturating individual ARTCC sectors with too much traffic. ATC published the resultant "Preferred Routes" between many major terminals, and that's what they expect the users (airlines) to plan and file.
The "Pref" routes sometimes get changed. For years, PHX-SFO flights went up over LAS/BLD, and then up to BTY (Beatty) and up J92, and then westbound, comming into the Bay area over MODesto. Then, it changed. The stuff now heads west of PHX to PMD (Palmdale, CA) and then turns northward from there. Notice on the map that the GC distance PHX-SFO falls right inbetween these routings, so a fuel penalty is paid either way.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.