I've seen "somebody doesn't agree with the standard and we must support it" way too many times, and I've written JSON parsers because of this. (And, of course, it's easy to get some difference with the JSON standard.)

I've had problems with handling streams like the OP on basically every programing language and data-encoding language pair that I've tried. It looks like nobody ever thinks about it (I do use chunking any time I can, but some times you can't).

There are probably lots and lots of reasons to write your own parser.