This is similar to asking why `sqrt(-1)` is not defined. It is not defined if we're thinking in terms of real numbers, but if we expand our definition of "number" to include complex numbers, then it is defined, and it's `i.`
Similarly, we can define logarithms for all nonzero complex numbers (which include negative numbers). We have to make a choice which logarithm to take, since there will in general be more than one. This choice is called the principal logarithm of the number. The principal logarithm of `-1` is `pi i`, for example. The details are a little sophisticated and they're in the link below.
So, logarithms can be defined for negative numbers, but we have to use complex numbers to do it. If we insist on sticking with real numbers, then justaguide explained well why we won't be able to define logarithms for negative numbers.