There may also be a massive threshold between doing shuttle speeds and doing the sort of speeds a scout needs. In TOS, any stupid old freighter could do warp two - but no more, despite supposedly being gigantic in comparison with the smallest craft capable of this feat. OTOH, small "scouting" craft such as those flown by Cyrano Jones or Carter Winston were larger than the theoretically interstellar shuttles, but only accommodated one person.
Even in TNG, the fastest we've heard a shuttle go would be warp 4, for the unseen Type 9 in "Resolutions". Something keeps the shuttles from being as fast as the slightly larger runabouts (at least warp 5). It might be something as simple as nacelle size - but also something more complex, involving stuffing the interiors with proportionately more machinery (as the Danube
seems to have less crew-accessible space inside than the Type 6, proportionally).