Oh so many reasons. There's:
Companionship!
Monkeys make great buddies.
Excitement!
You never know what's going to get broken with a monkey around.
Jealousy!
Oh man, people would be so jealous of your monkey.
Envy!
Pretty much the same thing as jealousy.
Revenge!
A dish best served... monkey.
No no, a REALLY expensive one.
You don't want some used thrift store free monkey. You deserve the best of everything. And anything that's the best is really expensive. That's why it's really expensive: it's the best.