I don't know how people afford to have children. Or houses. Or two cars. Or foreign travel. Or weddings. Or shop in Waitrose.
Could it be that people just live differently to me? Is that even allowed?
Well the government gives money to the majority of carers for children. I'm fairly comfortably off but we still get nearly 2k per annum child benefit. I don't believe a similar allowance exists to help smokers.
People who have houses sometimes bought a long time ago when they were very cheap, or inherited them, whereas inheriting a life's supply of cigarettes from a relative is perhaps less common. You can lease cars for a surprisingly small amount a month, but that wouldn't really work for cigarettes. Waitrose do offers all the time, whereas that's not permissible for legally sold cigarettes. I guess lots of people pay for holidays and weddings on credit, and in theory you could do that for cigarettes, so there's that.
But it's not just about living differently, it's the fact that most adults don't have £70 a week disposable income and we know that the people on the lowest incomes are disproportionately likely to smoke. Most of the population are also not in the position where rearranging one's life a bit will yield a spare £70 a week. So I can quite see why OP would wonder how so many people are able to afford it.
TBH, as someone who lives in a community with high smoking rates, the answer to OPs question is that people aren't paying a tenner a pack. There's always someone who does fag runs. So you get it from them. This is what my low income, tobacco loving family members do.