You'd be surprised - for a number of musicians it's not about sight reading notation and playing fom there, it's about developing musical talent and variations and adding in formal notation reading | writing later.
I've known a family of three siblings who all played in the state orchestra in their teens and only one was a fluent reader, the other two were extremely good musicians that practised hard and either very slow worked through written music note by note, or, once familiar with a piece looked at a page of music with a helicopter view, not reading" individual notes, but tracking off entire blocks .. like seeing a page of text w/out wearing glasses.
From the same music roots, some years later, Tim Minchin- admittedly more an Entertainer than a pure Musician - but none the less someone that made their name and living from performing original and relatively complex music on stage playing piano and singing - also doesn't read music: