As already mentioned, it's limited by the improvements available - it would be rather weak if it healed less HP and you ran out of improvements before you did anything useful. It does need to compensate for the need to rebuild said improvements. But mainly I think it's deliberately asymmetrical to help AI attacks - human players improve most or all of their tiles; AIs do so less consistently. So a human attacker will have fewer improvements to pillage and so benefit from than an AI. I suspect the improvements to Denmark and cavalry are incidental, with the AI buff the primary purpose.