Why Cura flattens first layer of round objects?
All models must be sliced horizontally into layers in order to be printed, and each layer is flat. That is simply the bottom layer. Since you are printing a cylinder, you could always rotate it by 90 degrees, and stand it on its end (before you slice it, naturally). You should always consider the best orientation for printing any model.

Also, if you print a cylinder on its side, the slicer will need to generate a lot of support structures to support the lower curved surfaces. By printing a cylinder on its end, no support should be required.