I'm not going to get my roommate anything for his birthday
Why don't you want to get your friend a birthday present?
Because I don't like to waste money on gifts.
It's understandable not to get a present if you're low on cash. Usually, people will understand if you can't buy them something when you're having financial difficulties. However, if you're not having financial trouble then it's rude not to get a gift for your roommate and they may resent you.
That's cool since everyone loves me and everyone is tight with me.
That may be true now, but if you treat people like this it will change. Maybe you can do something for him instead of spending money.