Hi,
This has been done on purpose.
The description is inside the form of the product page.
If you add a product in the description and it has itself an add to cart button displayed, you'll have the form of that add to cart button, inside the form of the product page, and this won't work as you can't have forms inside forms in HTML.
However, to add products on the product page, we do already have a mechanism for that.
You can add the products in the "related products" setting of the product and you'll get a listing of these related products on that product's page.
Now technically, you can add the product tags you want in the product description and they will be replaced on the frontend. You just don't have the interface to generate the tag itself, but you can generate it in a joomla article and copy/paste it in a product description.