From a pilots point of view, I totally agree with you. I'd never close a flight plan before I have landed. As you say, this is my lifeline if something goes south after I've left the frequency.
However, many other pilots do just as you say. I do not agree with them, but I can still understand why. Many GA-pilots feels that the flight plan is just a pain in the ... And when they finally file a flight plan, many are afraid that they should actually forget to close the flight plan when they have landed. This have happened, and say that you have forgot to switch on your phone after landing, and the 30 minute period expires, the ARCC will try to establish contact with you and when they isn't sucessful, it can be a very bad day at the office..
Many pilots are afraid for this, and therefore they might tend to close the flight plan already in the air when it is fresh in their mind so that they won't forget it.