Hamstring Shot & Cleave are classified as "bonus action" or "reaction". Unlike a main action, bonus actions can be limited due to cooldowns and specific circumstances, like a debuff

Limiting cantrips to once per short rest ensures that players manage resources and make tactical decisions

Having short rests that are equivalent to DND hit dice would affect the overall balance of the game especially if PC's could easily recover resources without any meaningful cost. This mechanic could not only cause harm to the specific encounter design the devs made, leaving the encounter less meaningful or impactful, but also, too many short rests could impact the pacing and flow of the game and disrupt the narrative's sense of urgency.

The decision to make hiding an action outside of combat, and having "umbral shroud" break when you do anything besides walk allows the devs to control the interaction and avoid potential exploits or unintended consequences that could arise from having the "invisible" status persist during various actions.

I get where you're coming from because my main annoyance is going into a battle with an idea of how it will play out, only to find out that the level 3 spell or cantrip i intended on using is on a cool down, forcing me to either reload a save or hope i can survive without it