One of the biggest things I've found to make a dynamic combat is to have the enemies start in multiple locations. If you just have everyone smash together into a large melee, then movement becomes largely irrelevant. With multiple directions to deal with, the party either has to separate into several smaller skirmishes or leave the "back rank" characters open to attack. In these cases, the PCs might even need to take an OA to switch between them (e.g. the paladin wants to take on one big bad, but a cleric with Spiritual Guardians wants a bunch of minions instead).
Sad, but true. Some players just can't see the potential reward worth any penalty or risk. Of course, some DM can't think of worthwhile rewards to go with said penalties either...To a lot of players, "you can but at a penalty" is functionally "you can't."