Once the plane is down they have no further piloting needing doing and need to help with the evacuation. Witness Sully Sullenberger on the Hudson -- he was the last person out of the airplane, having already assisted all the passengers in evacuating and then gone back to perform a thorough sweep to ensure that no one else remained.
Given that the inflight safety demonstration is on video now, what’s the critical safety role?
Another less "critical" role, but still not something I want a pilot doing, is dealing with disgruntled passengers.
They also ensure that rules are followed and that people, e.g. in an exit row, understand what they need to do. If pilots couldn't begin preflight until passengers are seated and buckled it would take forever to depart.
Speaking of boarding, flight attendants also deal with seating and baggage issues. Again, it would delay departure.