It doesn't take more than one or two strikes before the first enemies in most games disappear in an explosion of smoke or blood. Sometimes it's satisfying, other times its empty. Most often though, it's a simple task, and one we have to repeat many times over.
The reason is probably as straightforward as we need to understand how to kill. Both the seasoned players and the green ones. This reason also explains why the first enemies don't have complex behaviour. That would make trouble for the new players. The simplicity is sometimes alleviated by good visual design (like the classic look of the Dragon Quest slime or the splicers of Bioshock).
Another, probably more interesting thing to do, is to hide more interesting behaviour behind the player's increasing body of knowledge and skills.
Take the replacable grunts in the classic Viewtiful Joe. Punch them a bit, they die, but the entire system of the game is built around juggling the deaths of these poor sods, so that that cause more havoc in their demise.
Another interesting mechanic is, if simple enemies play the role of both chum and resource. Any game with a creature that is farmed for ingredients fullfil this role, but if the resource gathered is part of the overall gameplay loop, the harvesting makes the simple enemies feel like they are part of an eco-system.
My favorite mechanic though, is it simply just feels gooooood to kill a weak (or any) enemy. It doesn't even have to be the final blow though: Good old Tales of Phantasia for the snes is a delight for all 60-70 hours of the game to hammer endless amounts of fantasy beasts to the ground. The best example, though, is Resident Evil 4, where even the easiest enemy, is a treasure trove of fun ways to die (including the superb headshot-pumpkin-explosion) while also being a system for survival, where enemies can be incapacitated, slowed down, disarmed or simply murdered.
Not all these mechanics work in all games (and definitely not all at once), but the initial enemies could definitely be considered a chance to both let experienced players relax, provide immediate satisfaction in the battle system, and perhaps even be icebergs, eventually showing the full depth and breadth of the game.