Because it adds something that is not essential, so it is not "simple". If package A depends on package B with an incompatible license then it needs to provide a means to specify alternate packages. Arch follows upstream with best effort possible, so changing dependency could be seen as breaking that. However, pacman already supports choosing alternate packages during installation so technically they could add it, but how many percentage of users are needing that convenience? Not to mention the arch team will need to maintain said list of alternatives for every dependencies. If you want, you can add it yourself via pacman hook to prevent you from accidentally installing non free software, or write a wrapper for pacman, or use other distro like Parabola.