Because the only reason people need licensed media are for official projects, whereas those looking for foss/oss code want it specifically for that reason. People still actively look for those licenses when needed, but someone throwing up a photoshopped picture on Facebook doesn’t give a damn what license it has.

If code was shared in more open places (like forums of old) it wouldn’t have a license either even though it is technically foss until stated otherwise. That last bit is the gotcha and reason why those looking to legally cover their ass seek out these licenses for commercial/educational purposes but only care about open source software personally due to the hope it’s easier to find exploits in the code (or any other reason).