Why can I only zoom in so far to an offline map that I've downloaded to my phone?

When you download maps for offline use, the app downloads a very specific zoom range of map tiles. This is primarily due to space limitations with the offline map on your phone. With each incremental zoom level, you're quadrupling the size of the map above it which could take up more storage than desired. 

If you're interested in being able to access a deep zoom level, your best bet is to break the map up into smaller chunks. This will ensure that you're getting the level of detail for the specific areas you want without using up all of the free space on your phone's storage.