The problem is that a game aren't just a disk in a box. There is many layers to it now a days.
Who owns the right to the game? Who owns the IP? Who have they sold it to?
Lets say company (company A) makes a game (The Game). They sell it and earns on it and it is popular. Shortly after Company A goes bankrupt/dies/whatever. Now they sell off everything to pay off their debs, among those things sold are the rights for The Game, this includes all the rights for profits and the IP. Then comes Company B who buys it, because they like the game and want to make a sequel.
Now Company A no longer exists. So by what many now thinks that it would be okay to pirate it, since the company no longer exists to make a profit. Lets go back to the example.
Now Company B who bought the rights for The Game, saw it as a good business to buy the finish product, so they can support it and develop a sequel. In the meantime they are depending on the profit from The Game to get them though the development process to the sequel.
Now in the meantime people believes has become that the owner of The Game are no more and it therefore are free. They pirate it and shares it at will and the sales declines.
Now from Company B's point of view, they see that the profit disappears and interest in The Game declining, so they chancels the development of the sequel and eventual stops supporting The Game.
Can you see what you just did now? This are just one example on how things like this works. Just because the company there made it are here no more, then there are still someone out there owning the product and still need to make money on it since they pay for it.
It doesn't matter of it are an other game developer or if it is a game retailer branch who bought it, there are some one out there, who owns the rights for it. Nothing are up for grabs!
And those copyright laws someone started to mention, they are only based on private creations, like music, from some artist who didn't sell a given record though a studio. If there is a company involved, then it is a completely different chase.
So [TL;DR] Pirating are always illegal and will always damage someone down the line. It may even be yourself!