Because the Constitution says there are certain things the states may not do in interstate relations. It is the same reason that an interstate internet sales tax isn't allowed unless a multi-state company has a physical presence in the taxing state. The Founding Fathers understood why. You apparently do not. I'll trust their judgment over yours any day.
What is interstate about Indiana offering an incentive to an Indiana employer? I think you are the one who doesn’t get it. Whatever is not write not directly in the Constitution is left to the states or the people. Again, if you don’t live in Indiana it’s none of your business and if you do you have two choices, help to pass a law to change it or move to another state that agrees with you