There are several technical reasons why, none of which have been been subject to any technological advances in half a century.
1) There's a buffer factor where burning fuel adds a thousand degrees (or whatever) to temp of the air in the engine and steel / titanium / classified will melt several hundreds of degrees above that. Most all jet engines can only work with subsonic airflow. Supersonic aircraft use exotic inlet designs that are inefficient but can convert fast air into very hot compressed air. Somewhere around mach 2 to mach 4 the inlet air temp plus the heat of burning fuel will melt any metal turbine blade. You can pay a lot of money to get a couple mach numbers but fundamentally cheap steel gets you mach 2 and price is no object aerospace material tops out in the mid mach 3 range. True lab experimental materials might survive mach 4 temps, maybe. You just can't get a usable thrust to weight ratio inlet design that works above mach 4 or so.
2) Second aerodynamic problem is if you define "fly" as a lift to drag ratio better than a lawn dart, optimizing wing sweep etc for mach 3+ means its a truly awful performer below 5000 feet or so. Its hard to make an aircraft that actually "flies" above mach 4. Space shuttle L/D ratio was around or below 1:1. Essentially things flying thru the air above mach 4 don't fly in the sense of wings producing lift, they're ballistic trajectory like a missile or bullet, don't bother slapping wings on them.
None of the above can be solved with faster computer cycles. Titanium still melts at the same temp, etc.
Designing an air breathing propulsion system for a wide speed range is difficult.
Rockets are much easier after a certain point. McDonnell Douglas and Paul Czysz had interesting projects. If you want manned reconnaissance, a Mach 6 air launched liquid rocket powered lifting body would probably be the next logical step from the Blackbird and would not even be super hard. With rockets you don't have the inlet problem at all and they have excellent thrust to weight ratio.
> "The Air Force decision to retire the Blackbirds in 1990 is based on several factors. In congressional testimony, Air Force Chief of Staff Gen. Larry D. Welch identified the increased survivability of reconnaissance satellites, SR-71 vulnerability to the Soviet SAM-5 surface-to-air missile and the cost of maintaining the SR-71 fleet."
[0] https://www.latimes.com/archives/la-xpm-1989-04-09-op-1582-s...
A modern spy plane however would have to be faster than the SR-71, which I don’t believe would be safe against modern SAMs.
I don't believe anything worthwhile about current capabilities has been declassified, it's all conjecture by people looking at the X-37 and similar systems.
http://www.projectrho.com/public_html/rocket/spacewardetect....
There's also the outside possibility that faster manned aircraft exist, but remain classified [1].
[0] https://en.wikipedia.org/wiki/Boeing_X-51_Waverider [1] https://en.wikipedia.org/wiki/Aurora_(aircraft)
And in general, now that we have the technology, the SR-71's role is much better filled by unmanned craft like satellites anyway.
Stuffing a man inside one of these deathtraps and getting him home safely adds orders of magnitudes of difficulty. A better question might be, why would you want to send a person up in something like this, if you could possibly avoid it? It would most definitely be awesome, but it would come at the cost of billions of dollars and possibly human lives.
Also physics is a real bastard. Air resistance increases with the square of velocity. Even small gains over the SR-71's speed would come at a very, very high cost in terms of fuel burn rate, etc.
Additionally, if someone had indeed broken the record in some black project or the other, chances are they would keep mum about it rather than calling Guinness Book of Records.