Oh where to start.
First of all if I'm spamming an affiliate product I have to pray to the gods that the affiliate program or company will actually pay me when time comes. So I'm promoting, and I don't have guarantees that I'll be paid for my efforts in the end. It makes me sick!
Second of all, most affiliate programs don't allow you to promote their crap by spamming. If you do, they may refuse to pay you, or ban you. And that's another thing, they may just ban you regardless. They may ban you because you made so much money that they'd rather keep it than give it to you. So they ban you and give you some bullshit reason, and then cry me a river. Or maybe they ban you because their cigarette didn't light today, whatever, they don't give a shit, fuck you.
Third of all, what happens when the affiliate offer you're promoting dies for some reason? All that promotion you did is wasted now. Granted, you can be smart and make sure you have access to all the links you build, so you can change the destination URL if something happens. Still, risky.
Fourth of all, when I get a customer they can be my customer for weeks, months, years, forever! But when I get a customer for an affiliate program, that's their customer, not my customer. They reap the benefits of forever, not me. Fuck that shit son!
Fifth of all (relevant to the point above) - Of course, you may get an affiliate program where you get paid for everything they make from the traffic you send in perpetuity. But go to the pesky little privacy policy or whatever and find the part where they say that "We Can Change The Rules On You Whenever We Fucking Want And Go Fuck Yourself You Piece of Affiliate Marketer Crap." When you find it, realize that they'll drop your affiliate commission % whenever they want, they'll cancel the recurring payment thing whenever they want, and go fuck yourself!
Sixth of all, even if you make money and even if they pay you, you're gonna wait for that money. With what I'm doing the money comes immediately, before I even do any work (excluding the free balance). I prefer to get my money NOW, not later.
Seventh of all, affiliate marketing is like everybody and their dog does it. I'm a magician, I do special unique incomprehensible shit

(ok this isn't a valid reason, but I get to fuck around a little bit on my own journey thread goddamnit

)
There you go...
If you need more reasons, let me sleep on it and I'll come up with more.