Partial literacy is all you need for YouTube or video calls. Plenty of hearing people can read well enough for that too, or to find what they need at the store, but can't read well enough to e.g. summarize the main points of a newspaper story. I've seen estimates that something like 20 - 40% of Americans are functionally illiterate in that way. For the Deaf, it is even higher.

A speech and language therapist tells me that spelling and reading is hard for deaf children because we match phonics to text but they don't have access to phonics, so they have much lower literacy levels without specialist help