Because that dropdown list is used to set other things but the language, such as date/time format, currency, default paper size... It has been considered important to provide a UI that clearly distinguishes between the display language and regional formats.
Even if the language is the same, I take for granted that "French (Canada)" differs from "French (France)" in several regional formats aspects. You know better than I do as regards "French (Belgium)", but Ubuntu assumes that the existence of a separate locale means that there is something that differs. Hence all the available locales are displayed on the regional formats list.
Because that dropdown list is used to set other things but the language, such as date/time format, currency, default paper size... It has been considered important to provide a UI that clearly distinguishes between the display language and regional formats.
Even if the language is the same, I take for granted that "French (Canada)" differs from "French (France)" in several regional formats aspects. You know better than I do as regards "French (Belgium)", but Ubuntu assumes that the existence of a separate locale means that there is something that differs. Hence all the available locales are displayed on the regional formats list.
HTH