You’re right ... there’s no reason.
Perhaps suicide, but why go through all the trouble? Think of all the potential points of failure: you might not be able to hijack the plane, you hijack the plane but the pilot lies about your heading and you’re not over the Indian Ocean, who will hold the passengers at bay while you attack the pilot - you’ll need an accomplice who also wants to die in the southern ocean, etc etc.
A plan to commit suicide by hijacking a plane and then crashing it into the southern Indian Ocean... kinda far fetched, imo.
If it was a hijacking there almost certainly was more than one person. Which brings us back to mechanical failure.
This just doesn’t add up.