Disc vs pc

I was told that if you buy the disc, you can access the game on pc as well. Of course you can imagine my disappointmentwhen that wasn't the case. I do like having a physical copy such as the disc but also like the options on pc. If we can buy a DLC code for both pc and console at stores and online, why can't we have the same option for buying on disc as well? They cost the same. Cant return disc for a pc/console code anywhere, so disapointed.