A single RAW can often be used directly in HDR software, and many tools already do this without needing you to make multiple JPEGs first. But creating several “exposures” from one RAW does not add new image information—it only remaps the same captured data.

That’s the key limit: a true HDR composite gains dynamic range by combining multiple shots exposed differently, so each frame captures different parts of the scene without clipping highlights or crushing shadows. One RAW only contains the range the sensor recorded in that one exposure. You can recover some shadows/highlights from it, but you cannot exceed what was captured.

So:

• single RAW HDR/tone mapping = useful for processing convenience and style
• multi-exposure HDR = useful when the scene’s dynamic range exceeds what one frame can capture

Also, cameras/software don’t always do this automatically because tone mapping is a creative choice, and many photographers dislike the typical HDR look or artifacts such as halos.