I think it's partly because humans have a tendency to impose conscious thought without appreciating the underlying structure. Taking bacteria as an example, they mostly "just" involve chemical reactions designed to convert food into energy, and therefore into more bacteria. There's no reason to assume or to project consciousness onto this process: it's almost required by the nature of chemical reactions. I'm not saying that bacteria are merely the equivalent of, say, acids and alkalines reacting to create salt, but they are similar enough.
I think in the same way, to give sperm consciousness is misleading. They are driven to the egg by just wriggling around until they find it (and, if they end up in the wrong place for whatever reason (*winks suggestively*), I doubt they are aware of that!), and when they find it the fertilisation process is driven by another chemical reaction.
What makes actual consciousness different? I have no idea. But it's still important to be careful to distinguish order deriving from, say, physical equations and chemical principles, from bona fide consciousness.