Slots are for capacity control. Even ATL with 5 runways has limitations. At 1 operation every 1 minute 30 seconds, ATL has a perfect weather capacity of 200 operations and hour, or 100 takeoffs and landings. Hence, ATL will peak at just over 100 slots per hour. With no slots, flights would far exceed the realistic capacity at an airport like ATL.
For the SAN/LGB question, LGB is slot restricted in real life where SAN is not. Therefore the 38 departures per hour at SAN represent full capacity while the 6 per hour at LGB more closely represent the capacity at a slot restricted airport. Also keep in mind terminal facilities at these airports. Just because a single runway can handle 30 departures an hour does not mean the airport has the terminal space to park 30 planes at once (an issue that will be addressed with the recently announced airport upgrade feature.)