why don't france merge with belgium to form a country?Watch
The north of Belgium speaks Flemish, and for a long time they have wanted further autonomy and some even want independence or a merger with the Netherlands.
Yes Wallonia, the French speaking part of Belgium, and France might share a language, but they still have separate cultural identities, history, and are happy running themselves. To put it in context you could make the same argument for why doesn't the UK and Ireland join because the vast majority speak English, but someone with even a small knowledge of the background would understand why they are separate.
Also Switzerland has four different languages, and has been going as a state for ages. They seem themselves as Swiss, not German, Italian or French.