How are BTC wallets hacked?

Basically. the wallets are not what holds the bitcoins per say... Technically all of your bitcoins are stored and dictated by the ledger. but its heavily encrypted so as a general thumb of rule only you who know the decryption key can use a wallet client to make changes with your wallet within the ledger.

now this is not entirely accurate but it should dumb it down a little to make the concept easier to grasp. hope that helped