How can Bitcoins be stolen?

I have been following the crypotcurrency for some time, especially BTC, and I must have missed out something because I believed the whole concept about a blockchain was to ensure no one can take possession of your coins. That the multiple checks of transactions was the core and could not be manipulated with.

But I must be wrong since Bitfinex had a hack and "they" stole nearly 120.000 BTC - How did this happen?

If the stolen BTC cant be traced and retrieved then isn't the whole bitcoin idea a little to weak to be a viable alternative?