We use pity as a noun or a verb but not an adjective. So is used before adjectives. You should not say:
*You are so boy.
This is ungrammatical because boy is a noun, not an adjective. The same is true of pity:
*It is so pity that you cannot join us now.
Since pity is not an adjective, this too is ungrammatical.
Instead of saying so boy, form an adjective from boy and say boyish. It would be correct to say:
You are so boyish.
In the same way, form an adjective from pity and say pitiful. It would be correct to say:
It is so pitiful that you cannot join us now.
In this answer, the * symbol marks an utterance as ungrammatical.
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"So pity" sounds like Doge to me because, as the answers explain, pity doesn't have a degree. very grammar. much confuse. wow, English. ;) I hope the article I linked explaining how to create Doge phrases will be helpful in understanding how so, much, many, et. al. are used in English (and maybe be interesting at the same time). I would probably say "It's so unfortunate" or "It's so sad" if I didn't want to use "such a pity".
– ColleenV – 2016-12-17T19:48:21.0472Pitiful is an adjective, but "It is so pitiful that you..." has a different meaning and is a bit insulting. – stannius – 2016-12-17T20:38:59.663
2@stannius There's also 'pitiable' - which is a little bit better to my ear, but really isn't an improvement on the idiomatic 'it is such a pity ...' at all – Au101 – 2016-12-18T00:22:56.507
You could say "so pitiful", but it would come off as mean or condescending. – OldBunny2800 – 2016-12-18T03:45:59.773