It's of course 100% possible, just not in a super quick or direct way. One common way is to simply listen to the song, tapping a shortcut every bar to add a project marker and then use the SWS action to convert those markers to tempo markers (make sure timebase is set to time in project prefs before converting, so the audio doesn't try to adapt to the tempo). You might have to adjust these manually for accuracy before converting them.
Alternatively, ReaGate can output MIDI notes at transients for you and you can convert those to project markers, and then tempo markers with 2 SWS actions. Could be faster and more accurate that way if you happen to have a steady kick track.
One more way is the amazing SWS/BR action: "Move closest measure grid line to mouse cursor (perform until shortcut released)." Assign a shortcut to that action and try it out, it's pretty intuitive.
Look around the forums, as I'm sure there are even more methods of achieving this. You're right though, it could be a lot more straightforward.
Just be very wary of any kind of tempo mapping if you have MIDI in the project...
http://forum.cockos.com/showthread.php?t=114766&page=2