Why is LSD a class A drug?

Was doing a bit of research and saw LSD is considered a class A drug. Also saw that there is currently no evidence of physical short term or long term harm caused by the drug. It''s also not physically addictive. How on earth did this come to be a class A drug, yet ketamine and spice are class B? Same question with magic mushrooms tbh although there are more risks with that. NB: I've never tried these drugs or any other drugs so I have an unbiased opinion.