There are several reasons for that. Balconies and patios are a harsh environment that many plants cannot cope with. They are often windy which contributes to pots drying and can burn the leaves of sensitive plants. They are often baking hot, as well as getting direct sun they can have heating effects from concrete slabs. Many ‘indoor plants’ have been taken from the forest understorey where they are sheltered from wind and received only part sun or maybe none at all. In some cases their natural environment is high humidity. You don’t get those conditions on the usual patio.
The second main reason is that potted plants, especially outdoors, are at risk in pots, particularly in small pots. Regardless of how much room the roots need the volume of the pot determines the reservoir of water available. In hard conditions that may be exhausted well before the next watering. Some plants cope with drying out others, including ferns, do not. Being in the ground gives a bigger reservoir and keeps them moist with less frequent watering.
It isn’t just decorative plants that may be at risk. People like to have culinary herbs in pots because you can have them in a unit and take them with you, or even if you have a garden have them close to the kitchen. This is fine for herbs like rosemary that do not like wet feet and will stand drying out. It is not good for things like basil that will wilt at the drop of a hat if it dries out or mint (and Vietnamese mint) that like to be moist all the time. If you are concerned that mint will escape in the garden and so must be in a pot use a big one, keep it out of full sun and water it religiously. You can’t kill mint they say. Maybe not but if it gets dry too much it will grow poorly and not be lush.