I have never understood why they did not use robots from the very beginning.

Because, as an earlier article quoted in this thread mentioned, silicon chips are radiation-sensitive, and if you couldn't get enough radiation-hardened electronics for the robots, you'd simply be adding to the pile of inoperational junk lying around the site.

Radiation-hardened electronics is VERY expensive, only produced in minute quantities, and is generally not available in systems that can run off-the-shelf software. It's mainly used in space applications, and even the space radiation environment is easier to deal with than a disintegrating nuclear reactor. If you're looking for a chipset that can run a pre-existing set of semi-autonomous robot software modules on a standard operating system while fending off globs of white-hot melted core, you're going to be looking for a long time.

I suspect they may try to get round the problem by simply throwing robots into the plant like Stalin's infantry into a minefield, hoping they can clear the corpses before they stack too high. There is a real risk of catastrophe from any such attempt.