Why hasn't anyone made a upgradeable gaming console yet?

People were already doing upgrades on the Playstation2 + Original XBOX + XBOX 360, even though the manufacturers wished they wouldn't and voided the warranty.... and ban them from their online gaming platforms.....

It's not that easy, take the XBOX series, it has always been a mix of custom and sometimes generic hardware packed into a custom box, with fans to keep cool.  The first XBOX360 had notorious cooling probs, despite being pretty big.  So you are even limited by the dimensions of the box you sell this upgradable console in.  

Since the cost of the new box is always mainly the cost of its development, you won't save any money, you might as well chuck the dirt-cheap plastic box and have all your new components in a new box.

They already do software and firmware updates.

Electronics have planned obsolescence built into them. 

Because they want you to buy a new one. They wouldn't make enough money with upgradeable consoles.