Why hasn't massive derivatives exposures at banks already led to disaster? I recently heard that Deutsche Bank had 
$72trillion of "derivatives exposure", which 
is many times greater then the entire German GDP.

Now as I understand it, derivatives are essentially 
just bets on the movement of assorted prices... 
so I am assuming that "derivatives exposure" means
the total amount you would have to pay out if
you lost every single one of your bets. The 
likelyhood of this scenario may of course be 
vanishingly small, but presumably with 
$72trillion of exposure, you wouldn't need 
much of a losing streak in order to leave 
you bankrupt.

So some people are saying - "ooh this looks 
scary, it could lead to disaster"... but my 
reaction is "ooh this is scary... but why on 
earth hasn't it led to disaster already?".

Now I know there were multiple 
disasters around 2007/8 already and derivatives 
may have played a significant role in that... but
I'm referring to, lets say, the past five years. 
How could Deutsche Bank have gotten to 
$72trillion of exposure without blowing up 
again? What kind of bets have they been doing
such that they haven't had enormous losses?

EDIT: FYI this question has a followup here.
While gross notional exposures are huge, net exposures at the banks are much smaller, on the order of 0.1 percent of gross exposures. Since most financial risk (but perhaps not operational risk) is proportional to net exposures this is a more sensible measure of total derivative risk at the banks. Since net exposures at the 6 banks with the largest derivative exposures circa 2014 were on the order of \$300 billion while their total assets were more than \$10 trillion, we may not have seen a disaster because the actual risk is appropriate to the scale of their broader operations. 

Here is an example of how this works. At time 0 a bank does derivative trade with notional of 100 with A and collects the offer (or ask) price. That is, the bank sells the derivative to someone who wants to buy it. At time 1 a bank does derivative trade with B with notional of 100 and collects the bid price. That is, they buy the derivative from someone who wants to sell it. Total (gross) derivative exposure is 200, two derivative contracts each with an exposure of 100. However, net exposure is zero, and the bank keeps the spread as profit for its trouble. There is likely some remaining risk in the form of operations and credit risk, but this is what the spread and initial and variation margin are for.