Can you just hack a system and steal their nuyen?
No. Except in very rare circumstances, nuyen is stored either on a certified credstick or in a bank account. Think about your online banking - even if someone hacked into your PC, using your password, they would still have to go up against your bank website and a whole other set of rotating passwords etc. to get at your money. Even then, there'd be a datatrail of which account the money was transferred to. I figure it works much the same in the 2070s.
Oh, and banks are almost certainly going to invest in the best Matrix security money can buy (that's how they get more custom!), which means milspec-level tech in some cases. And they aren't going to be afraid to Trace and Burn you.
I should mention this, most people have plenty of websites and store their passwords in a text document somewhere on their computer. I thankfully do not, and force my mother to remember her passwords because of this. Also if you've gotten into their computer with one password there is a very high chance that the password for the bank account could be much larger. Banks usually use a "security question" if your accessing your account from an "unknown" or previously "unused" computer. <.< So it helps but there are ways... .xD
