Why container terminates, despite I have an interactive docker exec command running?

I have the following container which I've created using the following command:

docker container create --name="my-service" ubuntu:latest sleep 120

When I start docker container start my-service this container it runs, then exits after 120 seconds, all OK.

Now in a second experiment I start the container, and while it runs I execute:

docker exec -ti my-service /bin/bash

Question

After 120 seconds the interactive bash terminates, because the container itself terminates. Why, and what is exactly the docker engine logic to doing this?