I believe they use the GPU for video encoding, while most Remote Desktop solutions are CPU-only. Also, I wouldn't be surprised if the encoding used by Steam is lossy, which is fine for games and fast-moving video but would be completely unusable for text.

Most remote desktop solutions don't even use video and instead transmit parts of the screen that have changed as still images.

When not consuming media content this approach is better because the fine details look more crisp. In video the finer details are limited by a bitrate & key frame frequency.