How can I add new tasks below the currently selected task?
You cannot. The Microsoft page for Adding New Tasks starts by saying "Select the row below where you want a new task to appear."
All other Google results give the same impression: it's simply not possible to add new tasks below the currently selected task.
Why is inserting above the default?
That's because you can always add tasks below the last line, since there are blank lines, but how do you add above the top line?
So Microsoft decided that Insert and Add Above the Top Task will use the same code and thus save them coding, debugging and adding the above/below option to each "add a task" operation.
That's a good Project Management decision; if you ignore the dubious User Experience. :-)