Why does poverty even exist in 'advanced' nations, such as Canada, U.S., and Great Britain?

I think the kind of poverty experienced in western nations is relative. It's nothing like the poverty of the third world. A family in the UK who are considered in poverty will have an income via social security benefits. They will eat, have shelter, have clothing, and most will have TV's, mobile phones, DVD players, Xboxes etc.

A lot of it also depends on how parents manage their income. If they choose to spend a lot of money on their drug use for example, their children may well be in poverty in that they don't have enough to eat or suitable clothing. This isn't because they aren't receiving enough money only that it is being spent on the wrong things.

It really all depends on how you measure poverty and what criteria you use.

Because it doesn't matter how advanced a civilization is, there will always be slackers unwilling to work. As Susana S said, the poor in the nations you mentioned definitely live at a higher standard than the poor of third world countries.

How do you think they become wealthy.....not by sharing~ someone always has to suffer, that's a given

There are too many bugs in the system. Aside from that people who are well off have the uncanny ability to forget about the less fortunate and to blame the less fortunate for their problems. Ultimately the human race is selfish.