Since treating poultry as meat is rabbinic in origin (even though it’s not meat) and is therefore somewhat of a stringency/fence, why wait the full 6 hours after eating chicken before eating milk and not just 3 hours or even less? Thanks.

Shalom, First of all let me apologize for the delay in answering your question. There have been some technical problems with the site, and I only now got your question. Your question is a very good one. I know this because it appears in such classic works as the Pri Megadim (M.Z. 87,1) and the Pri To'ar (87,14). Nonetheless, they (and all the halachic sources) rule that we treat chicken like meat in the question of waiting after it before eating milk. There is an opinion that the prohibition against chicken with milk is from the Torah, and indeed Tosafot (Hullin 104b "Off U'Gevina") explains that this is the reason we are strict not to eat milk directly after chicken. Even though some major opinions hold like this, the overwhelming halachic view is that the chicken and milk prohibition is only Rabbinic. According to this view, we will have to say that the Rabbis saw fit when decreeing chicken and milk as forbidden, to equate it as much as possible to other meats in order to strengthen their decree. Blessings