You think so because you have been living in a multi-lingual environment. It's not that safe to assume that a person knowing Hindi would understand all those regional languages. Knowing Hindi certainly helps to understand 2-3 regional languages spoken around you, but biggest factor is that interact with different speakers everyday. Take Bengali for example, there is no way that a Hindi speaker can understand Bengali naturally. It's just we who get feeling like that. Same can be said about other languages too.

Some languages have same script while some have common lot of words which makes it easy to understand. Maithili, the language spoken in my native place, can be understood with ease by Hindi speakers, can be spoken very roughly, but when you try to write proper sentences with somewhat decent sentence formation and grammar, it's gets difficult to do so.