I was in a college level big-band back in the 80's. We sometimes had rehearsals which were solely sight reading practice. The director would hand out a chart and we would play it once and then give it back to him, never to see it again. This would go on for two or three hours at a stretch. Sight reading is a skill like any other, and gets better with practice.

I couldn't sight read my way out of a paper bag now. I'm quite out of practice.

For what it's worth, I miss playing in a band with real players.