The insecurity isn't in the design of Bitcoin. It's people passing their Bitcoin to a third-party wallet which holds it in trust and provides equivalent currency in, say, dollars or Euros. It's like giving your cash to a bank without a vault or security on the doors. The cash works just fine (or else the criminals wouldn't want it)... but if you're giving it to third-parties to hold who are unable to secure it and unable to have sufficient funds or insurance available to cover any potential loss, that's a risk.
Your own Bitcoin wallet, that's forever and secure for as long as you keep it secure. Putting those coins into a foreign wallet is an act of trust in that third party and numerous incidents have proven this to be quite a bad idea.
The places hosting these exchanges aren't secure, aren't audited properly, don't have intrusion detection, prevention, insurance against those kinds of intrusions (which would almost certainly demand security procedures and audits as a condition of insurance), etc. Banks are legally required to do that, and hold a certain amount of funds in trust in case of problems, and all sorts of other regulations. BitCoin exchanges don't, maybe the cash-side of their businesses but their BitCoin sides don't.
And that's the problem. Bitcoin are valuable, but the risk is in the exchange for other things of value. As soon as you try to cash them out, you are in the hands of a third-party who has to take your Bitcoin and give you something tangible or cash in return.