Because they don't know any better and/or think wadges of spam is normal LOL.
I have to admit that I learned the hard way too. I used to get 120 totally unsolicted spam emails per day (and rising), then found my email address using Google on just two websites. I changed my email address and have had none of that for a least three years - only the semi-solicited stuff from purchases, which proved easy to stop.
In fact I was bit hasty and probably mislead you. It seems there was only one hit for that "exact address" (this post). However, you only need one hit for the spammers engines to find it so your advice still stands.
Please come back and let us know the outcome.