Drugs desensitise people to life's "regular highs" which makes them seek out stronger, more stimulating experiences in their lives, which makes them want more money (since these experiences tend to be expensive), which makes people more competitive in the workplace/marketplace (and more likely to be dishonest and prioritise short term gains)... This makes it harder for non-drug-users to compete in the workplace/marketplace.

Legalising drugs would essentially force everyone to use drugs. That's the same reason why drugs are banned in professional sport.

From my experience, drugs make people less altruistic. Although it does also make them more social (but social != altruism).