Modern networking follows the Seven-layer OSI model which is successful due to the principle of separation of concerns.
The value of separation of concerns is simplifying development and maintenance of computer programs. When concerns are well-separated, individual sections can be reused, as well as developed and updated independently. Of special value is the ability to later improve or modify one section of code without having to know the details of other sections, and without having to make corresponding changes to those sections.
TCP/IP is layer 4 ("Transport")
The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.
SSL/TLS is layer 6 ("Presentation")
This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.
In other words, it is a conscious design choice.
Also, TCP predates SSL by several decades. To wit, SLL keeps changing because weaknesses are found; meanwhile, TCP has remained mostly the same. The OSI model allows easier adoption of changes by keeping some things constant while varying others. If we built encryption into TCP, then it would be a far less stable protocol.