Because cryptography is a specialized knowledge. Most curriculums doesn’t even include cryptography as core topic in their Computer Science degree. You can have a look of the MIT’s computer science curriculum. Cryptography is instead embedded in the elective class of Fundementals of Computer Security (6.1600). That’s also why DevSecOps instead of the previous DevOps. It’s just simply boils down teaching and learning cryptography is hard. It’s still too early to expect a typical dev to understand how to implement cryptograhy, even with good library. Most doesn’t know compression and encryption doesn’t mix well. Nor they understand the importance of randomness and never use the same nounce twice. They doesn’t even know they can’t use built-in string comparison (==) for verifying password hashes which can lead to timing attacks. Crypto lib devs who understands crypto add big scary warnings yet someone will mess something up.
Still, I will strongly support academics adding basic cryptography knowledge to their curriculum, like common algoritms, key lengths, future threats, and how fast the security landscape is moving, just for the sake of the future of cyber security.