Have you ever downloaded an MP3 from a friend? Used a YouTube movie download tool? Installed an adblocker in your browser?
This is the price of entry for digital distribution online. If your assets are good, then a lot of buyers will pay for them for real, and a bunch of them probably will end up not actually shipping them in a game anyway, so it evens out. If you are morally opposed to this happening, your only option to make sure that doesn’t happen is to not make things available digitally.
And, by the way, this pre-dates online sales; grocery and convenience stores have significant “shrinkage” as part of their numbers, and have had since the beginning of time. The benefit in digital is that, when someone steals a pack of gum, you can’t sell that pack of gum to someone else, but at least with digital sales, you still have the ability to sell the same bits to someone else. The draw-back is that, once the bits are out there, someone else might also sell them. But the problems with trying to ship a game based on stolen assets are pretty bad, so that’s why legitimate asset stores actually end up dominating the market for legitimate buyers.