Because as soon as one wrote or said it (or using any other form of communication), it would mean nothing. Somebody may receive it but then has to interpret it. But one can only interpret something correctly when oneself has experienced it and even then most of the time it's still difficult to interpret our ways of communication correctly, because our means for communication are simply inappropriate and often misleading, because of its cabability of transmitting only very few content of experienced phenomena.
Suppose I would proclaim being a sotapanna. And now? Whose turn is it now to believe or not believe it? Yours and in the end it keeps on being "not for sure", whatever you'll believe. Suppose I proclaim not being a sotapanna. And now? Same thing.... just a waste of time.
I think one shouldn't give much attention to anyones supposed attainments. One would be better to recollect the Dhamma in ones own mind and keep on practicing.
Just a few thoughts... don't mind me
best wishes, acinteyyo
edit:
a quote from the book "Gifts he left behind - The Dhamma legacy of Ajahn Dun Atulo"
Above & beyond words
A well-read layman was conversing with Luang Pu, saying, "I firmly believe that in our present day and age there are not just a few monks who have practiced to the point of reaching the paths, fruitions, and nibbana. So why don't they make their knowledge public, so that those who are interested in the practice will know of the levels of Dhamma they have attained, as a way of giving them encouragement and hope so that they'll accelerate their efforts to the utmost of their ability?"
Luang Pu answered,
"Those who have awakened don't talk of what they've awakened to, because it lies above and beyond all words."