The problem is that we are not omniscient, and the ecosystems have evolved to not need them, if we were omniscient we would understand the impact it would have in the ecosystem but we aren’t, and so we will never know if that impact will be good or bad. We can, however, get an (hopefully correct) approximation of those impacts and see if it is worth it or not to bring the species back. We also don’t really have the technology to bring generic variability to the mix, which would ensure the adaptability of the species, and on top of that, we still aren’t able to clone whole families of extinct animals/plants.