Game balance. Enemies don't necessarily have class levels that correspond to ours, usually having fewer abilities but more HP and often more attacks. This helps with action economy, allowing the DM to put fewer enemies against a PC party of 4-6.
For example, Thugs (CR 1/2) make two melee attacks and have 32 HP. If they were actually "level 1/2" fighters, then they'd have one attack and like 4 HP and you'd need like 10+ of them to have any chance against players.
