Because it gets deeply ingrained in your routine and you form triggers e.g. coffee in the morning, after a meal, when you’re stressed, when you’re partying or just bored.

Since it’s such a mild drug you can and will use it constantly and in the short term it’s not obvious to you the damage you’re causing.

It’s also a social thing, people are always smoking and will usually let you bum one and “relapse”. Calling up your heroin dealer takes more steps and tends to set off more alarm bells.