That's because the standard SMS protocol does not account for read receipt transmission. Some carriers have it worked out in their messaging apps but that likely requires that both parties be on the same carrier. Your best option if you need to know if the message was read is to use a 3rd-party communication protocol such as Hangouts, Whatsapp, IMO, Signal. Those use data rather than SMS and I believe all those (and others) will show if a message has been read.
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