long term stock donation vs sell stock and cash donation

I am missing something. People keep telling me that it is more tax advantageous to donate the stock as opposed to sell the stock and donate the cash. hypothetical scenario of 10k of stock turns into 50k worth of stock(so 40k gains) The way I think about is if I donated 50k of stock I would avoid capital gains tax, but wouldn't selling the 50k of stock and then giving that 50k also avoid taxes? That 8k(20% of 40k) would be deductible so it wouldn't matter. I feel as if I am missing something obvious. Thanks!