This is not a rules question.
I have been playing a druid and one of the things that doesnt make much sense to me is the wild shape ability. Specifically in how it functions. It makes sense that the stronger you are the longer you can hold a form, the further away from your original size you can get, and even able to support different ... biology? I can even understand the abilities being withheld until you are strong enough to use them like pounce and grab.
But why does a druid gain the ability to become some things and not others?
Why can they take the form of an owl or a bear, but the owlbear (which is a magical beast) is beyond them?
Everything the owlbear is came from forms that the druid is able to produce.
The druid can change into stone, air, water, and fire, but somehow dragons, fey, and vermin are beyond those capabilities?
The only thing I can think of is the druid's connection to nature. It would explain the elements, plants, and animals. It also explains why "unnatural" magical beasts are not possible, but surely there are natural magical beasts. Vermin should also be considered a part of nature, but they are not allowed.