Hi nebpor,
I work for the airline and actually asked the same question when I was lucky enough to have 2 seats to myself and wanted to put the armrests up to make a sort of bed.
The reply I got was that the backs of the seats are wider to improve passenger comfort. This has had the effect of reducing the gap between the seats to less than the armrest width so the decision was taken to prioritise having a slightly wider seat over the armrest going fully up. It also reduced wear and tear on the IFE cables present in the armrest.
The 777 also has seats where the fuselage tapers in that have non-moveable armrests. This is because the armrests are padded in order to comply with the 16G protection that the interior on-board has to be certified to.
Hope that helps.