So you’re loving your new Magento 2 store when all of a sudden you realize, your simple products have started turning themselves into virtual products. Magento 2 automagically freely converts simple products to virtual and virtual products to simple based on whether or not the product has a weight.
All you have to do to turn those virtual products back into simple products is provide a value for weight. When you go in to edit the product, though, you may find there is no weight attribute on the screen!
Look in eav_attribute for the weight attribute_code and get its attribute_id (entity_type_id is probably 4). Look in catalog_eav_attribute where attribute_id is the one you just looked up from eav_attribute and set apply_to NULL. Reload product edit page, and you should see weight now