Why would you complain about this? Is it foolish for the driver to do that, yes. Would I prefer that over some idiot passing aggressively and potentially unsafely? Yes.
As others said, probably a insecure driver who wants to be safe and not anger/upset you.
I tend to be a fairly conservative driver when dealing with passing bikes, but when its very clear I do pass, leaving plenty of room and not slamming on the gas aggressively when doing so, but sounds like I would have passed you in this situation
Did you consider trying to "Wave her on" at a good point to pass? I have occasionally pulled far over and waived drivers on in similar situations.