Why is my hair so dry?

I only wash it when it needs to be washed about every 3 days, I never blow dry only if im in a hurry and usually only twice or 3 times a month, I only straighten it when im going on a night out which is once a week sometimes I dont even straighten it I just put in some hair serum, I use moisturizing conditioner, and I only use a small bit of shampoo, and every so often I leave olive oil in my hair over night, I never ever brush my hair unless im straightening it, and I trim it every few weeks but its still dry and full of split ends. Its very thick and wavy. Why is it in such bad condition?