The only way to get a misdemeanor charge (that was dismissed) removed from your record is to obtain an order sealing and destroying your records pursuant to California Penal Code section 851.8. In terms of why you have to go through this process when your case was dismissed -- I suppose you'll have to ask the California Legislature why they enacted this process. However, that is the process whether we like it or not. You'll have to file a motion requesting the courts to seal and destroy your arrest record pursuant to 851.8. It doesn't matter if you were actually arrested. If you were charged with a crime that you were never convicted of, 851.8 is the process by which you clear your record.
The standard for 851.8 is high. The burden is on you to prove that you deserve to have your record sealed; it's not the government's burden to show otherwise. An 851.8 motion does not apply to all cases where the charges were dismissed. It is to be used in cases where you are factually innocent of the charges, which is different than being not guilty. "Not guilty" could mean the DA didn't have enough evidence to convict you or there was enough evidence to convict you, but the charge was dismissed in the "interests of justice". Factually innocent, which is the standard required in 851.8, means that you are completely innocent of this charge.