It is not where they are looking.
Old Boeing Driver asks is there is anything else to add, past that the A/C took off and contact was lost. Yes, there is. We know it took off, we know it had a competent flight crew, we know it had lots of fuel, we know it climbed to altitude, and we know it was on its intended course.
We can reasonably conclude that the A/C was on autopilot. These guys are not likely to be hand-flying that thing in the gloom of night on a long-haul.
We also know that vast amounts of assets are out looking at the water surface. Plus, lots of small boats out there, fishermen, diesel-fuel tankers, junks, coastal freighters, all kinds of stuff. We know that an aircraft impacting water is going to bust up into a vast number of parts, and a lot of those will float around. Yet, nothing is found.
That tells us that it is not where everybody is looking. So: where is it? Getting past murky reports of radar contacts in other places at different altitudes, you do have to conclude that the more logical flight path is the one the autopilot has it set on. That A/C heads North - to land. From that inference, either the A/C continues onward on that autopilot until it runs out of fuel, then crashes, or the autopilot is interrupted and the A/C goes into an uncontrolled descent and impacts terrain. We have learned that an impact into terrain at high speed will leave a small crater - in the case of United 93, only 30 feet across. Are you going to spot an impact crater in the jungle of Southeast Asia? Or does the jungle simply swallow it up?
In which case, you may never find it. Ever. Hard for us to appreciate, with our fascination with technology, yet still quite plausible.