If you want to kill yourself - you just fly the plane into the ground during landing or takeoff. It is way too elaborate to get to the no mans land and then fly 5 hours and crash somewhere.
Lookup Silk Air 185. Pilot went to lengths to disable CVR and FDR so that there was no record of him incapacitating the FO and directing the plane into the ground. He had an insurance policy he took out that day. Some people don't want to have a wikipedia article saying they murdered 238 people. So, fly it into the middle of no where and no one will ever know. It's working so far.
I'm not at all an expert on the topic but I would guess that people who are willing to commit suicide are emotionally troubled and may not act in predictable or rational ways.
