It wasn't, and arguably is not in any way constitutional. This became a thing mostly after 9/11. Terrorism and drug laws were significantly expanded in scope. For years the public was looking the other way - distracted by fear - while the government began openly breaking or bending the law, almost without any concerns.
During that time, many behaviors became routine (with or without actual laws backing the behavior up) that have extremely questionable legal standing, and typically zero moral standing.
Asset forfeiture rates were actually significantly down in the years immediately following the September 11th attacks. It wasn't until 2006 that annual forfeiture recoveries reached the rates they did during the 90s (at which point, yes, they increased sharply — nearly quadrupling in just two years). [1]
As for the constitutionality of asset forfeiture, you'll have to take that up with SCotUS, who've (unfortunately) rather consistently disagreed with you.
