You are right, a longer antenna would be advantageous, but there are diminishing returns. Generally speaking (very generally), for an omni directional antenna you would have to double the length for every 3dB's of gain.
If 3dB is 3' long, 3dB is 6' long, 9dB is 12' long, 12db is 24' long etc. By the way, those numbers aren't correct, but just a simple way to prove a point. Again, 3dB doubling the length isn't entirely true, but I hope that the concept makes sense.