Why can't one switch back to the default mode on Magento 2?

The default mode is neither here nor there.

The Magento team really wants you to use either Production Mode on your live site or Developer Mode during Development. The reason why Default Mode exists is to have something that would at least run reasonably when deployed directly from the downloaded code without further intervention.

By running php bin/magento deploy:mode:set you have identified yourself as capable and not needing the default mode. As there is no reason needing to go back to default mode Magento did not provide a way for you to do so.

The default mode is there only to install Magento 2.
It should not be used for development or production.
After installation you can only switch between dev and prod.

Source: Alan Kent @MagentoImagine2016

As a windows developer, in developer mode, Magento wants to create symbolic links to the static assets, but uses the Linux command and not the windows command. This creates a lot of errors in the js and css files.

To switch back to default mode, open:-

app/etc/env.php

And change:-

'MAGE_MODE' => 'developer',

Back to:-

'MAGE_MODE' => 'default',

And re-deploy static files again.