Just wondering, why cant the upgrades, aircraft and money be transferable to next airport? Its easier that way instead of redoing the whole thing again

you can sell your aircraft and receive golden planes for it. this way you can transfer money to another airport.

I fell into that trap when i converted 50 gold planes at the beginning for INN (500,000) only to find out i couldn’t do that either. Frustrated was not the word. But hey, my INN got developed real quick!
However in reality even airports in group ownership as managed as separate entities so have to generate funds on their own.
With the devs trying to make this as realistic as they can do (I still shake my head when seeing the BA380 at PRG), it follows that each airport should stand on it’s own two feet and you start from scratch.