There is no evidence that Franklin ever said this. Well, no evidence aside from that quote being attributed to him on tee shirts hanging in the gift shops of 90% of breweries and that this misquote is trotted out every time a journalist is assigned a beer story which he researches only as deeply as Wikipedia.
To be fair, there are so many sources that use this quote that it is easy to understand when someone believes it to be true.
Besides that, it is such a lovely idea and who else would have said but our beloved founding father who also told us to fart proudly.
Franklin did write, "Behold the rain which descends from heaven upon our vineyards, there it enters the roots of the vines, to be changed into wine, a constant proof that God loves us, and loves to see us happy."
It is sort of the same sentiment and it is understandable that someone some time back was trying to remember that thing Franklin said about some alcoholic drink and, without double-checking her source, came up with this misquote.
So, a few understandable mistakes led to everyone deciding to misquote Franklin. What's it matter? Well, it does not matter in the grand scheme, I suppose. This little mistake is not likely to profoundly affect the course of human history or anything so drastic but, still, can we agree that it's always better to stick to the facts?