And these callsigns have to be de-conflicted with every other operator in the world, both military and civil. So the application process is lengthy and also your callsign is liable for rejection.
My company's callsign was changed about 6 years ago to avoid confusion to similar sounding ones. The new callsign was changed to one with only one confliction - a US recconaissance unit. I wasn't too happy about sharing the same airspace as them during a war!