Why do smokers throw their cigarette butts on the ground?

Can't they just carry a personal ashtray,
or if that's too difficult, put them in an empty cigarette pack?
Why is it that every single person throws them on the ground, or in flower beds, or down the drains?
In all my life, I have never seen a smoker who carries a personal ashtray, regardless of age or gender.
And why do they smoke even when it is clearly marked as a non-smoking area?
It's not just one or two people; is it some kind of group mentality?
It makes me so angry every time I see it.