Does it mean that no one can outsmart those hacker?
No. It's just that hackers are always one step ahead. Anything created by man is not absolutely perfect and someone can sit somewhere to find the error and a means to exploit them.
Why is it that their major targets was on well know sites?
Because only well known sites have what they looking for. Their mantality is if you risk big, you earn big.
Does it mean that no one can develop a stronger security measures to outsmart those hackers or does it mean that most of the hacks that is happening are from Insider? Or maybe someone who is already in the system are leaking out their information to hacker?
Some may involve insider job but some are not. Like I said before, anything man made is not free from error. No matter how much security you developed, someone can dedicate their whole time to study the system and discover the error. If you're looking for someone who's dedicated and patience, then think of hackers. They have this special characters.
Again, how do you trust the people that are being employed to work with most of the exchange or gambling site?
You don't just hire random persons, aside their experience you can do some background checks about them.
What are the strategies to ensure that the staff they are hiring is trusted and can not compromised their duties or works, because it's always said that only those who are Inside that can know the way about the system.
Division of labour. There are sensitive sections with sensitive data, not every staff can have access to where such data are kept. Only trusted staff should be given such access with a clear and detailed consequences attached. Hold them responsible for any problem that arise from the section they controlled.