Why can't you use table salt instead of pool salt in a salt water pool?

You can as it is the same thing The only drawback may be in the Case of iodized salt and what ever they put in table salt to make it flow easily.

I believe its the anti-caking agent that's added to table salt, also I think buying the pool salt would be more cost effective than a large bag of table salt (do they sell large bags? maybe at sams club or Costco)

The anti-caking agent may be magnesium carbonate - main constituent of the rock mineral dolomite & chemically similar to the calcium carbonate that is the primary constituent of chalk and limestone. So although it's much slower to dissolve in water, it would eventually increase the hardness. I don't know its scale-forming properties, but I think they are very low.