It’s not that reading is literally the only possible way to get the same kind of linguistic input to the child. I’m sure listening to an expert storyteller, watching a puppet show or play, listening to someone reciting poetry, having a thoughtful conversation with an adult while looking at some wordless artwork, singing songs together, etc. can be similarly stimulating.

The thing is: it’s hard to find actors for a play in the child’s bedroom at night. It’s hard for parent to come up with 200 different well-plotted stories at the appropriate level with visual props. Etc. But after 2 years of daily practice I can read a picture book decently (including funny voices, etc.) while half-asleep and consciously thinking about something unrelated with half my attention.

The kind of logical reasoning, grammatical variety, vocabulary, etc. used in books is not literally impossible to imitate with some other medium. It’s just a lot more work and in practice not really done.