* Many agencies - for example, the IRS, Medicare, Social Security, FEMA, agencies that provide loans, or any other service - have lots of information on individual citizens.
* Many agencies have information or make decisions that would distort markets if leaked - it's release must be carefully managed - such as the departments of Treasury, Labor, Commerce; or the Federal Reserve.
* The Department of Justice has lots of information pertaining to lawsuits and investigations that must be kept secret.
etc.
It could also make accountability harder because it would encourage avoiding paper trails whenever possible, even when employees believe they are acting within the law.
Is FOIA the right amount of openness? I don't know, probably not, but it's definitely better than public-by-default.
2) two people have tried to kill him in the last 6 months
Have you filed a FOIA request?
Other parts of the government are good at responding to FOIA requests, but it's generally inversely proportional to the value of the information.