Answer:
Because it is very abrupt, and you don't want to just end an essay that way because it doesn't make the reader want to believe what you wrote due to how informal it is. you want to have ecleast 3 sentences in you conclusion as well. You want to add a little more information to your conclusion, like recaping what you wrote in your essay to put that information back in the readers mind in case they forgot what you had said before.
