It is more than cost. FPGA also have security advantages. An ASIC can be reverse-engineered. A FPGA can be 'programed' by software at boot. So someone capturing

It is more than cost. FPGA also have security advantages. An ASIC can be reverse-engineered. A FPGA can be 'programed' by software at boot. So someone capturing the FPGA while it is offline learns nothing. That might be why the USAF doesn't go into panic mode when a fancy fighter jet crashes somewhere. Anyone grabbing it might only find blank FPGAs, with their as-used programed layout either deleted or hidden behind strong encryption. Equip your jets with ASICs and every detail of your avionics might be at risk.