Because you build up tolerance to it. Unless I inject a lot of h, which i would if i had enough cash (which, surprise, I dont have), i dont get high. You just use enough so that you dont get dopesick.
When i dont shoot up, my family actually starts thinking im using because i feel so hellish. Its just a short term solution that turns into an uncontrollable problem. It ****ing sucks not being able to control your own body, and sometimes half your mind.