Because, RB, there are evil people in the world who will take advantage of the fact that Jews are more generous then anyone when it comes to being charitable.
It’s worth noting that the halachas of tzedakah don’t only pertain to giving the money. They also concern how the money will be used. Which is why, nationwide Vaads have a nationwide database to ascertain that the money is being put to proper usage. Giving money to someone who’s just shaking a pushka because they heard Jews give money on Friday afternoons, and then that person goes home laughing, is sadly, not fulfilling the mitzvah of tzedakah.