Why not? Losing that level of capability, and the ability to iterate upon it, seems like terribly poor strategic planning in the past - much in the same way that ending human space after landing on the Moon was also quite poor strategic planning. Reviving the SR-71, in much the same way that human spaceflight is also being revived, seems quite logical - especially because in the case of the SR-71, in contrast to rocketry, there is nothing remotely comparable in the commercial domain.

Near the top of this thread, there's a lot of debate and discussion about what we must have 'secretly' concocted. Some think that the various sighted UFO/UAVs might be secret US tech. But I see no reason to think this is the case, in no small part because things like the decision to retire the SR-71. You don't get from A to E without going from B C and D.

There's also other practical reasons. If we did have anything meaningfully advanced then it'd be being used, however grossly inappropriate it might be, in places like Iran. No chance the current administration holds anything back for the sake of some strategic edge in a possible future conflict. I also think secrecy in modern times is not especially realistic. Everybody has eyes in the sky, to say nothing of probable increases in human level spying and the like. And so forth. Basically the idea of rebooting the SR-71 would be precisely with the aim of eventually fulfilling these sort of 'fantasies.'