Many deaf people do prefer sign language as an accessibility option over reading captions, even if the interpreting can be hit-or-miss.

Also, in India, many hearing-impaired people know only ISL.

This is also true in the US. People pick up enough written English to get by most of the time, but it's often quite broken and clearly a second language. I know hearing impaired people native to the US with substantially worse English than the average European.

Just so you know, "hearing-impaired" implies that a person has a flaw whether a person is born with it (it is natural to them) or impacted in later life (hearing-challenged).

Most non-offensive way to refer to a group of people without perfect hearing is "hard-of-hearing or deaf".

Sign languages is nothing like spoken language and so reading a spoken language is hard.