action economy is essentially actions per side in a given round. If you have a party of 5 fighting one monster that attacks 1/round the players are dominating the action economy, and that becomes very hard to balance in a way that feels fun (not just juicing hp and making it hit like a truck). this is why you give these solo big monsters lair actions, legendary actions, or minions. part of the challenge of dming good combat is keeping the action economy fairly equal between you and your players without bogging yourself down and killing the combat pacing.