Because you basically can make an infinite amount of anything & they aren't "one of a kind" anymore. I personally don't care as long as they aren't hacks (Volcanion is legit, ribbons, JPN region, not a powersave date, etc) but I do understand and respect people that don't want clones.
They're really useful though, like I could breed a new Onix everytime someone asked for one, but I can get the same exact results by just cloning the original, it saves a lot of time. If someone missed the Mew event / a Volcanion, which only has one official release ever (in Japan mind you) etc they're still able to get one off someone here.
It does take away from the "uniqueness" of the Pokemon.. but it's a necessary evil in my opinion.
They're really useful though, like I could breed a new Onix everytime someone asked for one, but I can get the same exact results by just cloning the original, it saves a lot of time. If someone missed the Mew event / a Volcanion, which only has one official release ever (in Japan mind you) etc they're still able to get one off someone here.
It does take away from the "uniqueness" of the Pokemon.. but it's a necessary evil in my opinion.