No one considers them to be absolutely secure. Rather there is a calculable cost to subverting the network, currently measured in the millions of dollars and growing. Having the cost of reverting a transaction be $X,000,000 is sufficient for most purposes.
There are plenty of organizations that, if they wished to mess with bitcoin, could easily afford more computing power than the bitcoin network (yes, even though that is "bigger than any supercomputer".)
Included in these organizations are ones (such as the US government) that have been accused by some of bitcoins proponents of being actively attacking bitcoin.
So why should we consider confirmed transactions to be secure?