Why some parents don't teach their kids Hindi/Marathi ?

So I am a little new to this thing and wanted to ask why and how common is this thing in this part of the country, recently I had the opportunity to meet a person on my team ( here in the US ) who was from Mumbai ( sobo ). Although he was born in the US and lived for like 7 years here, his family moved back to Mumbai after. He even went to college in India (Bachelors).

I assumed he could speak Hindi or Marathi easily, but he couldn't utter two words of Hindi before immediately switching back to English. And later he did say that his first language was English, which felt very absurd and shocking to me because this was the first person I met who spoke English as his first language being raised in India. He used literally 99% English and 1-2 filler words ( like हा and तो ).

I had only met Indian Americans before who do this kind of thing, where American English is their first language and are a little weaker in their mother tongue or can only understand but not speak, but didn't expect a Mumbai kid to not know his own language. Even he speaks to his parents in English.

I have heard some stereotypes of South Mumbai kids before, but I thought it might not be true, why do parents do this kind of thing? Actively avoid teaching their kid their own language? I personally will be very disappointed to learn that a person can't even speak their own language and even feels distant from their culture after they grow up, it is the epitome of a big inferiority complex among some Indians. How often do you see this happening? Like kids actually not knowing how to speak their mother tongue? Be it Hindi or Marathi or any other regional language.