Or alternatively, why didn't they use the warp engines that are clearly attached to the sides of the shuttle?!
They could be power units of a kind but not warp nacelles, per se.
Where you gonna fit a M/AM reactor in those things anyway?
The speed and capabilities of the shuttles vary based on the script.
Most episodes seemed to suggest that that the shuttles were warp-incapable. IIRC, and without checking, the tech manual suggests some of the shuttles have limited warp capability. It could just be that whatever system Picard and Crusher were traveling to had "restricted subspace-space" and didn't allow in-system warp travel unless in an emergency so the shuttle was dropped off at the edge of the system and the Enterprise went about its mission.