One of the problems you're facing is that combat is fairly abstract. In the real world, if I'm concentrating my efforts on stabbing Baron Zemo through his black, black liver, then one of his underlings will have an easier time flanking me and stabbing me through my righteous gallbladder. But in D&D, little things like facing, or even the number of opponents attacking you, don't really matter (except when they do because of special circumstances). And let's face it, hit points are part of the problem. Combat is designed to be settled by the attrition of hit points and if you spread your attacks around it takes longer to defeat enemies and results in PCs getting attacked more often. As the rules are written, it just makes more sense to concentrate attacks, except for maybe some situations where your goal is to control movement or other aspects of the board.
I don't know of a good solutation that wouldn't require a lot of house rules and making combat overly complicated.