I can tell you why!
The undercurrent is so swift at times, (and yes, even at the designated swimming area at McFarland), that it will pull you right under & you won't resurface until you've made it to 7 mile island!
Especially if there are barges running up or down the river, or the spillways have been open.
I personally know of 2 grown men (on separate occasions) who were swept under and drowned.
Both of their bodies were found at 7 mile, beaten & battered by the current & rocks.
It's Dangerous! Even for the most advanced swimmer!