1) most contracts worth implementing in Ethereum are fairly complex
2) even given great developers, bugs are inevitable in complex code
3) the budget of the contract-makers' security team MUST be smaller than that of the hackers
You quickly see that if the chance of a bug is nonzero, "smart contracts" don't make economic sense. If you have a $100k contract, and you spend $5k on security (which would absolutely destroy most companies' margins by the way) you'll be facing hackers that are EACH willing to spend up to $90k or so. Let's say all the experts in this example world are $200/hr. You spent 25 expert-hours on security. But you're being hacked by people who spent 450 expert-hours on hacking you.
With that in mind, would YOU want to use a smart contract? Spend 5% of the contract value instantly on security, and risk losing 105%? This isn't a normal loss by the way, where you can prosecute someone or sue somebody. No, this is the instant, digital theft of the entire value of the contract, to an anonymous digital address where it will be quickly blended in with hundreds of millions of dollars of similar thefts a month.