All Tantras and Agamas assume the form of a dialogue between Shiva and Parashakti and as far as the reasons are concerned i have till now found two of them.
And, both of them are from Scriptures.
In order to depict the importance of Guru
Guru Shishya Pade StitvA Swameva SadAshivah | Prasnottara
ParairvAkaistantram SamavatArayat ||
SadAshiva himself simultaneously assumes the roles of a Guru (as Shiva
as prakasha) and Shishya ( as Shiva's own vimarsha Shakti), and by
initiating a question-answer session thus propounds all the Tantra
A Swachhanda Tantram verse quoted by AchArya Bhaskara Raya while commenting on a verse in NityAshodashikArnava's 1st patala.
This verse's import is that, in order to show the world that all knowledge is essentially Gurumukhi, Lord SadAshiva propounds all Tantras in the form of a dialogue between he himself and Sri Devi.
To emphasize the fact, that all Shastras, Agamas are fruitful only when learned from one's Guru, all Agamas assume the form of a dialogue between Shiva and Shakti who assume the respective roles of a Guru and Shishya.
Another explanation is also found in Scriptures and which is as given below:
To reiterate the fact that all Scriptures are essentially Shiva-Shakti's creations
Shivena Paraya ShaktyA DvAbhyAm Kritsnam Samudhritam |
VAchyavAchakabhAvena DvAbhyAm Brahma PrakAsitam ||
All Shastras are created by the duo- Shiva and ParAshakti. They
together,by assuming the forms of an asker and an answerer, there by
reveal the supreme (Brahman).
Agama SAra Tantram
And the fact that, its Shiva (who is of course never separated from Shakti) who propounds all Shastras, by assuming different forms and at different times, is mentioned in many Agamas (for example in MahAnirvana Tantram's 1st chapter or in KulArnava Tantram's 1st chapter).
So, this is the 2nd explanation as to why all Tantras are always in the form of a dialogue between Shiva and Parashakti.