Negligent behavior. Bitcoin implement bank-level security, so anything stolen is negligent behavior. The vast majority of articles about wallets being stolen is when bitcoins were held in a third party online wallet, which was hacked. Keeping bitcoins in someone else's wallet defeats the idea of decentralization and owning your own money.
As for whether it's as secure as conventional currency, even more so: no one else has access to your money, unlike a bank account. In other words, no one can withdraw your bitcoins without you explicitly sending them.
As for whether it's as secure as conventional currency, even more so: no one else has access to your money, unlike a bank account. In other words, no one can withdraw your bitcoins without you explicitly sending them.