Question: Why do people use...

High gain amplification of low voltage signals is something that tubes are good at. High current low gain amplification is something that transistors are good at.

Can it be done the other way? Sure, but it isn't easy and will cost you.

I haven't used this setup (yet), but the idea is to combine the power and low frequency abilities of a ss amp with all of the attributes tubes are known for. Seems like a great idea to me and I plan to try it at some point. To reverse the set up and have a ss pre and a tube amp would miss the point.

I've auditioned all setups and still found Tube preamp to tube amp most appealing sound by far. Tube preamp with SS amp is the runner up.

SONY ES9000 SS preamp with Bryston 9BST SS amp
Audio Research SP-11 tube preamp with Bryston 4BST SS amp
ARC sp-11 tube preamp with McIntosh MC-240 tube amp
Mark Levinson 23.5 SS preamp with Sonic Frontier tube amp
Mark Levinson 23.5 with Nelson PASS X-150 SS amp
Marantz 7c tube preamp with Sonic Frontier tube amp
Marantz 7c tube preamp with Classe DR-3 HVC SS amp
Counterpoint SA-5000 with Lafayette KT-550 tube amp
Counterpoint SA-5000 with SET EL84 tube amp

I agree that most of SS amp has better low frequency reproduction but there are many tube amp can also produce good low frequency reproduction. I don't think BASS would an issue in tube gear as long as your amp is within spec.
However, tube amp are not recommended for Subwoofers because of the constant high current demand.