As has been said, the recipient is not obliged to send read receipts. When I was working, one of the PAs in the office demanded that people sent read receipts to her E-mails. This was soon jumped upon by management when they realised how much space was taken up by her E-mails on the server. As a consequence, everybody was given a limit on how much space they could have on the E-mail server. Some thought it was a wind up, but they soon realised it wasn't when they came in one morning to find a lot of their E-mails had gone!
This link may give you some idea as to why:
https://practical365.com/real-world-case-read-receipts/