You are actually misunderstanding the verse.
The verse is in fact the KEY to unlocking the understanding of the Prajñāpāramitā sutras - Heart Sutra, Diamond Sutra as well the Mahāprajñāpāramitā Sūtra.
Here is an alternative English translation (You Zai) of the verse from Mulamadhyamaka Karika from the Chinese translation by Kumarajiva
All dharmas that arise from causes and conditions,
I call them emptiness.
They are also provisional names,
And also the Middle Way.
The Chinese text
What Nagarjuna is saying, is that because all dependent originated phenomenon are due to causes and conditions, by their very conditionality they are impermanent and without self. As stated by the Buddha.
"When this is, that is;
This arising, that arises;
When this is not, that is not;
This ceasing, that ceases."
Therefore because things are impermanent and without a true lasting self. He call them 'empty'.
However the term 'emptiness' is also just a convenient label, because it does not fully explain the complexity of the phenomenon and all the causes that made up something. To say something is 'empty' is that it ultimately lacks an underlying 'essence', but does not mean that it does not exist but that it does not truly exist.
This is therefore the Middle Way between Eternalism and Annihilationism. To be annihilationist is to say nothing exists and that there are no causes for anything to occur. This isn't the definition of emptiness as used by Nagarjuna. It is because there are causes, there are outcomes, but because they are made up of causes, there are also impermanent, hence is not Eternal.
So whenever you come across a text like the following:
Form is Emptiness, Emptiness is form.
Your should consider the meaning of emptiness.
Form is dependently generated, Dependent origination is form.
Form is impermanent, Impermanence is form.
Form is without self, Without self is form.
So the question would be, why did Mahayana Buddhist Nagarjuna rephrase what is already a known Buddhist concept into 'emptiness'.
It because of what they perceived as the reification of Buddhist teachings in the Abhidhamma by schools such as the Sarvastivada (one of the ancestors of the Theravada), recall that even in modern Theravada teaches that Nibanna is unconditioned, a deathless dimension, and that the Arahants are perfect etc.
Nagarjuna denies such permanence and eternalism.
This is why it is stated:
There is No Wisdom, and There is No Attainment Whatsoever.
Because wisdom is itself conditioned (by the practice of the Noble Eightfold Path), and therefore wisdom too is impermanent. Hence the fruit of Four Stages of Attainment (Stream Entry, Once Returner, One Returner, Arahantship) are impermanent too.
It is not saying that there wisdom does not exist or the fruits of the contemplative life does not exist. They do exist, but like everything else are impermanent and not self.
Nagarjuna is breaking our attachments to the attainments within the very Dharma itself! Even Nirvana itself is impermanent! The end of suffering as is suffering is caused and conditioned and is impermanent!