Why is the Earth uninhabitable?

plays a thought-experiment 

I want you to imagine that, instead of being a perplexed mostly-hairless primate reading a blog, you're the guiding intelligence of an interstellar robot probe. You've been entrusted with the vital mission of determining whether a target planet is inhabitable by members of your creator species, who bear an eerie resemblance to H. Sapiens Sapiens. To gauge the suitability of the target world you've been given an incubator that can generate decorticated human clones — breathing meat-machines with nobody home up top. When you get to the destination you're going to transfer them to the surface and see how long they survive. If it can make it through 24 hours (or one diurnal period), congratulations! — you've found a potential colony world; one so hospitable that a naked and clueless human doesn't die on their first day out.