Why is UDP message-oriented, while TCP is byte-stream-oriented?

JDP is message-oriented. The packets handed over by the sender’s UDP to the application are delivered down to the IP layer after adding a header. It does not merge or split, but retains the boundaries of these packets. This means that the application layer sends long packets to the UDP, and the UDP still sends them, that is, one packet at a time. TCP is byte-oriented. “Flow” in TCP refers to the sequence of bytes flowing into or out of the process. The meaning of “byte-oriented flow” is: although the interaction between the application and TCP is one data block at a time (the size is not different), TCP regards the data handed over by the application as just a continuous unstructured byte stream. TCP does not know the meaning of the transmitted byte stream. TCP does not guarantee that the data block received by the recipient application and the data block sent by the sender application have a corresponding size relationship, but the byte stream received by the recipient application must be exactly the same as the byte stream sent by the sender application