The same reason we don't recommend regular expressions for parsing HTML.
To me, this is quite different.
The OP has an internal variable containing a (CSV) string and wants to quote the fields. It is really not like processing an HTML or XML external file, it is a variable within the program. The OP presumably knows how the string was generated and should presumably be sure of its content.
The string was probably generated within the program. And even if coming from some external source, hopefully the string has been verified and possibly untainted, maybe sanitized, whatever is needed to be reasonably sure of the content. If the string is coming from outside the program and not generated by the OP, these checks are necessary anyway.
Please note that I did not object to use the modules mentioned by haukex, quite to the contrary, but only to the advise "do not to try to quote the strings yourself". I believe that there are many cases where you know exactly what your data is like and where you really can quote the strings yourself. Sometimes, you don't need heavy artillery when a fly-swatter will do the job.