Military flight computing hardware isn't designed to carry everything you might possibly need, for a variety of reasons.
It is technically possible to input your flight plan and everything else you might use later from the in-cockpit keypad, but that means you will be either sitting on the ground for a long time or doing the aerial equivalent of texting while driving.
So what typically happens is that the military commissions a custom bit of software to translate the output of a common flight planning program into the specific format that aircraft uses. Then they carry that file to the aircraft and upload it.
Military avionics now has a ridiculous amount of required data, with dozens of different kinds of navigation points and a bazillion radios and sensors and weapons system modules.
So in order to write the custom data file creating software, you need the hardware spec. And you need to translate data from the flight planning software's format into the specific form that the flight computer expects. As you might expect, they never quite match up, because all the software is divided up among a gazillion contractors, who aren't allowed to publish a common standard or share code because some stuff is classified.
So it's not a matter of loading all the radars. The code to load even one radar probably hasn't even been written yet, because the contractor is still working on the module for the radios.
This problem would be much less severe if the military could hire software pros directly at anywhere near fair market value.