Automakers solved this problem many years ago, by making cupholders that are deeply recessed instead of thinking that cups wills somehow magically stay in place with a very minimal indentation (see the "cup holders" on the backsides of glove compartments in 1970s American cars).

Airlines would rather have more people in economy than to include cup holders.

You do sometimes see cup holders on private airplanes, but in commercial aviation the efficient solution is to avoid needing them.

Adding decent cupholders to cars didn't make cars bigger.

Similarly, adding decent cupholders to airplanes is not going to reduce passenger space.

Commercial aviation could stick as many as 850 passengers on an A380-800 in it’s maximum configuration, but the most an airline actually gets is 538 because of business and first class passengers having more room. Increasing passenger space by even 1 inch or just a pound of weight per seat makes a real difference.

Better cupholders do not decrease space at all. Some (better) airlines even have them: they flip down from the seatback. They literally take no space at all. This is ridiculous.