I find your question interesting. I think I might. But I too don't want to hurt anyone, and the problem with criminal plans, I suspect, is they often go wrong (as if other plans didn't...), and you may find yourself having done damage that you could not have foreseen. For instance, you rob the bank and the teller has a heart attack, or leads the rest of his life looking over his shoulder, not trusting people he has yet to meet and would otherwise have come to trust. That IS damage.
So what to do, what to do!
About why more people don't... I don't know about the UK, but here in Sweden a great part of the homeless community are people who ought to be in psychiatric care and often beg to be, but are refused again and again - it's a national disgrace. Some of these people aren't capable of creating a plan. Others actually do kill somebody - it happens here shockingly often. For instance when our Minister for Foreign Affairs, Anna Lindh, was assassinated a couple of years ago - but that's just one example.
My guess is that more harmless 'customers' often do try but don't succeed as the police probably know their game and ignore them, they just make them return what they grabbed.