Here's why:

* Calderas form from explosive eruptions: Calderas are large, bowl-shaped depressions formed when a volcano's magma chamber empties and the overlying ground collapses inward. Mount Rainier has not experienced the type of massive, explosive eruptions that would create a caldera.

* Mount Rainier is a stratovolcano: Rainier is a classic stratovolcano, characterized by its steep slopes built up by layers of lava flows and ash. Stratovolcanoes typically erupt more frequently, but with smaller, less explosive events.

* Summit features: Instead of a caldera, Mount Rainier's summit features a series of volcanic craters and ice-filled depressions. The most prominent feature is the Liberty Cap crater, which is a remnant of a past eruption.

Therefore, Mount Rainier is not known to have a caldera, despite being a large and active volcano.