The problem is with plans you don't know how long it will take to get from arrival to bring at the departure gate. There are multiple opportunities for large queues and delays, and the cost of missing a plane is much higher than that of missing a train.

With a train it basically never takes more than 10 minutes from arrival to walk to your platform, so you don't need to waste so much time just in case there is a delay. Also if you miss it you might be able to get the next train for free depending on your ticket.