That is, because it messes with your muscle memory and makes it harder to predict, where the mouse cursor will end up on your screen, when travelling bigger distances. Generally speaking, the downsides overweigh. This behaviour aims to make it easier, to move further distances (on smaller surfaces), while allowing for precision, when it is needed. Mouse acceleration means: the faster you move the mouse, the bigger the distance travelled. This behaviour (which is mostly caused by the heavy mouse acceleration on mac systems) can't be turned off in the System Preferences.
Many people struggle with the mouse behaviour on mac Operating Systems. Tl:dr (' too long didn't read') version of this, can be found at the end of the post.
