The fact that the same approach is used across several species to implement the same functionality (the functionality being excrete the wastes and excrete the seminal cells) is a prove of how effective this implementation is.
If you look at the implementation, it makes more sense to slightly modify an existing system rather than inventing a new one. The existing system being the tubing used for excreting the discards.
In this way the existing structures are used more effectively than the case where the reproductive system was completely separate. Having two separate lines, one for wastes and one for semen, would cost more in terms of energy, and would also pose the challenge of how implementing the complementary part for the receiving side (the female).
Not that another implementation would be impossible, rather it would have to be extremely more competitive to be successful in the present world. We cannot exclude that alternative implementation were present but did not survive through the mass extinctions happened in the past.