If ants are better than spiders, why are there more?
The way Hive is played is that a lot of pieces end up trapped by the 'one hive' rule. Ants are very good at this. Once your ants are trapped, it's hard to win. Spiders are weaker pieces and get sacrificed easily. With too few powerful pieces, it would be hard to keep the game interesting. The boardgamegeek faq adds:

Aren't spiders just weak ants?
A spider is indeed strictly weaker than an ant. Some people are bothered by this and propose many variants to "fix" the "problem". Others are not bothered by it, and say "A rook or bishop is strictly weaker than a queen in chess, too." While more limited, Spiders are useful in many circumstances, particularly in the early phase while the hive is small. Learning to use your spiders correctly can give you an advantage, especially in a circumstance where you might have just used a soldier ant. If used correctly, you'll usually only use them once, and they'll give you the pinning power you need early on, without wasting your soldier ant pieces for when you'll really need them in the late game.