Ask yourself these questions:

• When you capture a city what do you have to do in order to hold onto it?
  
• When you try to hold onto a city what makes that attempt harder or easier?
  
• When you try to hold onto a city what resources of yours get tied up in the attempt?
  
• When you capture a city how is the city's morale affected by the provinces around it?
  
• When is the best time to capture a city that is undefended and is unlikely to produce a new defender (is there any urgency? Why?)?
  

Then ask yourself what freedom you have (what can you do) to try to make the answer to each of those questions be an answer you like instead of an answer you don't like.

Presumably because they don't currently have the ability to take those particular cities. Therefore, in the interim, while forces relocate or build up, they scoop the surrounding territory. That land they grab is still granting manpower & cash, depriving the enemy of vision and serves as a uav if enemy units step out.

A tactic I use to quick capture inactives is just to spray NG runners everywhere except the cities. Then build airfields in ideal locations to fly the main forces in later for quick takedown of the cities. Therefore they don't walk everywhere. Just line them up in advance. Also, unless the insurgent check has happened, I also tend to time city capture for just before day change to avoid the high insurgency chance of day 1. Means I need less manpower in the garrison and it avoids the inf being idle in a city where it could be capturing provinces & being productive. Stuff like this is why the map can look like you describe.