Caching helps if you visit several pages on the same site. But AMP might be optimized for scenario when you scroll through several AMP pages from different site

Caching helps if you visit several pages on the same site. But AMP might be optimized for scenario when you scroll through several AMP pages from different sites in the search results. Using an external stylesheet would only make loading slower in this case. Another problem is that on an unreliable connection external CSS can fail to load.