A computer is a state machine, with a number of potential states represented by 2^n (where n is the number of bits of storage/memory). As soon as n approaches some non-trivial number (which necessarily includes queued processor instructions, in addition to data!), the number of possible states rapidly exceeds the number of atoms in the known universe. There are simply too many permutations to test 100% of all cases for any program more complicated than "$x=2+2; assert($x==4);".

And that's just one constraint that makes the mythical "100% test coverage" practically infeasible. Another is: what tests the tests?