Every case is different. Yes, I know a few families who are 2nd or 3rd generation non-working benefit claimers and they need a great kick up the backside. Some people who are disabled feel the need and are able to work - most of these do have good family support and money behind them to make this possible.
With mental illness, you can be OK one day and the next day find it difficult to put one foot in front of the other or, as someone with ME has told me, for every good day you go through 2 or 3 bad days to make up for it. I have a neice with ME which came on 4 years ago. Before that she was very sports minded and incredibly healthy now, one day of going out with her mates will mean that the next day she will find it hard to walk round the house without falling over. She could not even do the journey to work, let alone manage a full day - hopefully sometime in the future, she might improve.
Some single parents are able to work - again, if they have a support network behind them but many do not. There is no one to have a child when it is ill (childminders will not accept sick children and many family members are also working and cannot take the kid. I know of single parents who have lost jobs because their child has come down with an illness and they have not been able to find care).
If you are single with no illness then there is no reason in theory to be on benefit especially if you live in a city/large town. For those living in areas without adequate public transport and either don't or can't afford to drive, their options are limited. Moving is not an option because no money = unable to move!
It is impossible to judge a whole section of society. Try living their lives and see if you could manage. Yes, there are alot of lazy bums out there but propaganda by the likes of Daily Mail and the Sun mean everyone gets tarred with the same brush!