Rehab is more expensive than jail and convincing citizens to spend tax money on drug addicts who chose to screw their own lives up instead of putting that tax money into schools is a tough sell for politicians. Drug addicts are typically a terrible investment with near zero return. Better to spend money on people who choose to live their lives in ways that better their communities.

"War on drugs" was actually genius marketing because it conceals the fact that the government is actually doing as little as possible as cheaply as possible by just chucking people in prison instead of sending them to rehab.