Why do some people can understand their parents' native language but can't speak it?

I have seen some Mexican kids replying in English to their parents speaking Spanish to them, they seem to understand Spanish pretty well,why don't they just speak it to their parents. I've also seen a Chinese waitress replying in English to her Chinese-speaking grandma at the restaurant. Why don't these people just reply in their parents' native languages ?

Is speaking a language harder than understanding it and replying in the language you are fluent at?

submitted by /u/No-Layer2025
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