Most of the answers already given are good, but there is a simpler one: It's too expensive.
It's so expensive that even the highly regulated and safety aware aerospace industry avoids formal verification for non-essential systems when they are allowed to.
Moreover, formal verification only makes sense in combination with certifying the software on some specific hardware, because it would be a pointless waste of time to run software that is guaranteed to behave well on hardware that makes no guarantees that it will execute the software correctly. Consequently, even small changes in hardware may force you to re-evaluate the whole system. So the maintenance costs of high integrity systems are also very high.
It's so expensive that even the highly regulated and safety aware aerospace industry avoids formal verification for non-essential systems when they are allowed to.
Moreover, formal verification only makes sense in combination with certifying the software on some specific hardware, because it would be a pointless waste of time to run software that is guaranteed to behave well on hardware that makes no guarantees that it will execute the software correctly. Consequently, even small changes in hardware may force you to re-evaluate the whole system. So the maintenance costs of high integrity systems are also very high.