I'm from one of the countries involved and the "being from a poor country" is just nonsense and insulting to those of us who were born there and lead honest lives. Also, these countries are no longer as poor as they used to be - in my country many people now have a better life than many in the Uk, including people with higher salaries than my Uk one. Even some unskilled manual jobs are now quite well paid.
There's still poverty like everywhere of course, but not all poor people are fraudsters. These people probably weren't even poor back home.
Why do they do it? I guess for the same reasons anyone commits a similar crime, for greed and what they see as "easy money". They want money but don't want to work for it and think they can get away with it.
In my country the legal system isn't efficient and can be corrupt, so they do get away with a lot, but the benefits system is a lot less generous than the Uk so it's not as profitable. Universal Credit money, when converted to local currency, would be worth a lot more than what they can get back home.
The shoplifting and pickpocketing - it's usually (not always, but almost always) people from...a certain minority background, as un-PC as that might sound. Stealing is a way of life for many of them, and these types are equally hated in their own countries by normal people. Except people there are wise to their tricks and it's more difficult for them. If caught they'd just get beaten up and the police are also not gentle with them either. Prisons there are pretty brutal too. In comparison they see the UK as a soft touch.
They probably don't view being arrested by British police and spending time in a British prison as a real punishment. So in short, they do it as it's profitable for them and because the risks are worth it to them. They also do similar things in Germany.
We (people from my country) are outraged by it too and don't want them in our countries either.