Why do heroin addicts get their life destroyed by using?

Not being a troll. I was thinking about this. If I calculate how much money i make from work, and dope makes me function at peak levels, then why do i fuck it up? if I make 50k a year and i only spend 300 a week on dope, but i am frugal, i don't go out , i don't have a family, etc...why can't people become functioning addicts?

i was one until i lost my job and then couldnt find work - yes, i lost it because of drugs, but i worked at a attorneys office where coke and alcohol were done by bosses and employees but if you did brown, no good.

i fucked up because i was young and stupid. i am now on sub and drug free...minus the sub of course. but i wonder sometimes, i day dream, could i become a functioning addict? i don't want to risk it, my sub doc gave me another chance, i did flirt with wanting to do Dark net but i dont want to be that person anymore. i want to be honest, help people, see a police officer in my rear view and just smile because i am all good. sub has really saved my life, i am glad i did it before i risked my life again.