[JohnBooty]

There's just never been a need. The SR-71 could already outfly missiles.

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.