The problem is that the level of control required for actual security prevents people from being able to do their jobs effectively. And if no one can get anything done, there's nothing to secure. So this leads to an environment where everything is oversecured by default, and bypassing the nominal level of security is simple, easy, and commonplace--sometimes even expected.
For instance, you don't have local administrator access on your workstation. But you have Visual Studio and its debugger, and can compile and run any source you can type in. You also have physical access to the machine, with its 5.25" removable-media drive. It becomes faster and easier to reimplement an unzip utility from a printed spec than to get 7zip installed on your machine. And the hand-rolled utility probably has a larger exposed attack surface than the open source program.
And for high security places why on earth would they allow people to work on laptops that are taken home every night an obvious security risk.
I am not dealing with TS stuff here. There are files on the government network which are confidential and having access does require a clearance, but I don't actually work with confidential data directly.
My bag was checked once in the 5 weeks I worked there - on the way in. The passwords I created for their new servers (containing metrics from the factory's build and test processes) was at one point walking around Cork in their admin's wallet. I used my own laptop (because OSX bleugh) plugged straight into their corporate lan.
But they did have a room which was out of bounds.
Nobody gets security right, however "high security" they think they are.
Oh and let's not get started on what the MoD thinks it's achieving in its immigration office.