Mostly, we don't take very high does of CS. It is very easy to train someone to do an IM injection on themselves.
With CS, there is no research that a bigger dose would have more effects. I would contend a small daily dose of no more than a few tablespoons a day would have very similar effects to an IV. I could be wrong, but it would be very easy for people to test this out and prove me wrong. I know many people that inject themselves with B-12 regularly.
An IV is much more difficult and expensive to perform. And as you mentioned you start having to buy bags of isotonic solution, etc.