in short: they reward time over skill. The best multiplayer games present you with a complete tool set from the get go, which means the most skilled players will generally win. CoD, and every game since that's included unlocks, reserves the best toys for the players who just play more often. The guns might only be a change of pace from the starter set but the unlocks like claymores, motion sensors, scopes, silencers etc all give an unquestionable advantage to the time invested.
Furthermore, having unlocks removes weapon pick ups from the maps, which means there's no reason to ever leave one area of the map or to even learn it properly because the game fundamentally rewards camping and punishes movement.