The problem isn't remotely controlling the plane. The problem is all the problems not actually doing it introduces. You now have more, many more, people that need to be involved because you now need to fake grieving families for 246 different dead people who never died (or never existed, depending on who you ask). You also need to ensure that nobody notices missing drone'd 757s (or worse, make sure that nobody notices the planes weren't 757s...), you need to ensure that nobody notices the planes are empty, and need to somehow get those empty planes into the airlines systems without anyone becoming suspicious of that, or trying to sell tickets for them. On top of all of that, you then still have a few live pilots who are witnesses you wouldn't have to otherwise worry about.

It is simpler to just find a few lackies to actually do it.