Probably a combination of the actual designs using metric units, some complexity of LCD manufacture determining the size of panels and - even if that leaves the height and width as nice round numbers, Mr Pythagoras messes up the length of the diagonal. (Unless it's a 4:3 screen...)
Why do retailers mark prices as $19.99 when in reality it really is $20 minus a penny (which is devalued here in Canada now btw).
Because 19 looks cheaper.
Similarly, 4.99 makes your brain ignore the .99, and look at the 4, and think, oh compared to the 5 inch phones this one is smaller sized.
They appeal to both the large size lovers and a bit smaller size lovers, because majority of public thinks after they have bought something impulsively.