But the point is that every army has certain weaknesses, and one of the lizardmen's is a lack of long range firepower. Putting something like fast moving, long-ranged archers into the list would be combating an obvious weakness, and this is something game designers generally avoid doing.
Look at the new WOC book for example: it the old hordes of chaos, they could take a vast variety of different troops for different situations. In the new book however, they have been limeted somewhat. However the troops they can now take are DEADLY in combat, but slow. If the army was given tools (such as lots of war machines) to counter this weakness, then it would change the dynamic of the army.
It's the same with lizardmen; we focus on movement and magic, and also movement and short-range shooting. A unit of archers with long range just wouldn't fit with the style of the army.