Someone explain to me why these corporations don't pay income tax

It's very simple... they've bought enough Congressmen to ensure favorable tax laws, and then they've moved assets around to qualify for whatever favorable credits their pet Congressmen established for them.

You too can buy a Congressman... just create a PAC which will repeatedly donate huge gobs of money to his reelection campaign. You'll see... he'll follow you about just like Mary's little lamb!