Which surely Cersei must know with spies and scouts. Even if she wants to respect the parlay thing, she must have enough men to just destroy them before the bulk of the Northern Army arrives since dragons aren't such a big deal anymore.
They said what's left of Danys army is roughly equal what Cersei has and she's got them all pulled back to protect her in the capital.
Edit: They wouldn't have known if both dragons would go with the fleet or not so trying to meet them on the road could go very very badly.