Posting this here hoping that someone with more knowledge can enlighten me about this. After going down a bit of the rabbit hole, I see that the SR-71's first flight was in 1964. It has held the record for fastest air-breathing manned aircraft[0] since 1976. What is the reason that given all of the technological advances that record hasn't been broken?
[0] https://en.wikipedia.org/wiki/Flight_airspeed_record