Because the probability of winning is so overwhelmingly low that even though the total sum you spend is small, it's a net negative for almost anyone. If you have a rare disease and need $1M in a month to cure you and die otherwise, then it might be a good deal.
But if it's a net negative that you don't notice...
I never buy lottery tickets or scratch cards (though I love sports betting and casinos), however my father does buy a lottery ticket every week.
He's retired, and has enough money to live fine - he owns the flat he lives in, he has enough money to pay for his £100/month TV/internet package, to smoke a pack of cigarettes a day, and to buy whatever food he fancies without thinking about what's cheaper to eat.
He has a choice of living like that and accepting that's how the rest of his life will remain, or spending a tiny amount each week and having a dream of becomming a millionaire. He's not an idiot, he knows the odds are against him, but his buying lottery tickets doesn't have any impact on his lifestyle, and it lets him imagine that one day he might win big.
He wouldn't notice any difference in his financial situation if he stopped buying the tickets, so really the only downside is that, if he keeps buying them over a twenty year period, the inheritance that comes to myself and my two siblings when he dies will be a bit less. But if you think that way, his quitting the lottery would make a far smaller difference than if he didn't subscribe to extremely expensive sports channels, if he didnt spend £40/week on cigarettes, and so on. I'd far rather see him enjoy his money than save it for my sake - I'll get by fine either way.
Sure, for some people the amount they spend on lottery tickets does take away from money they could spend on other stuff, but there are people who are well off enough to be able to afford the cost, without being so well off that winning the lottery isn't a dream they enjoy having.
The point being made by the parent and grandparent though is that whilst the expected value in monetary terms is negative, when people have a non-linear utility function, the expected value in utility can be positive.
I understand that. I'm just saying that people generally don't have that kind of utility function unless there are special circumstances (such as a disease). In fact most people's utility functions are sub-linear.
My point is that I do not consider the net sum negative. It is so small that it is inconsequential to my life. It does not affect my finances whatsoever. Only hitting the jackpot would, and that would be a definite positive effect to me. Like I said life-changing.