Nursery growers use the potting mix that is going to make their
plants grow the healthiest and fastest. That's how they make money. Replacing soil with something cheap for shipping purposes is labor intensive (expensive) and detrimental to
plant health. It is never done.
The idea that
plants should be repotted as soon they are acquired is a very common misperception and one of the most common causes of
plant failure.
New
plants should never be repotted until they have fully acclimated to their new environment (several months or more) and, then, only if they truly need a larger pot because they dry out every day or so.
I hope this clears things up.