Because of something called vacancy control. You can't raise rents on people in a unit. You could raise the rent of an empty unit to market rate (in Berkeley, in the 90's after rent control was changed).
EDIT: changed word "defeated" to "changed"
EDIT: changed word "defeated" to "changed"
The California Law called Costa-Hawkins phased out vacancy control in all cities from 1995 to 1999. After 1999, landlords have the right to set the initial rent, overriding local vacancy controls. (https://leginfo.legislature.ca.gov/faces/codes_displayText.x...)
In Nov 2018, California voters rejected Proposition 10 41%-59%, which would have repealed Costa-Hawkins and allowed vacancy controls again (https://ballotpedia.org/California_Proposition_10,_Local_Ren...). Berkeley, Richmond, and Santa Monica would immediately have imposed vacancy controls under their existing laws.
In Nov 2020, California voters again rejected Proposition 21 40%-60%, which would have allowed vacancy controls that limit rent increases to 15% per vacancy (https://ballotpedia.org/California_Proposition_21,_Local_Ren...).