Confederate casualties were 13,724, Union casualties were 12,410, and that was with the tough going (due to poor leadership) on the Union left. Lee knew he had to get out, McClellan's timidity permitted him to get away. One more day of that level of engagement would have used up the ANV.
One could make a case for your suggestion if Burnside's push to cross the bridge had been the whole battle, because the Confederates in that part of the battlefield were relatively few in number and chewed up a larger force for hours, but A) it wasn't, and B) A.P. Hill's corps arrived at the nick of time after Burnside's troops finally broke through -- but they had broken through.
Total casualties were very high, but even with a big chunk of the Union forces at McClellan's disposal being held in reserve, making it nearly an even matchup, and with the grueling, stupid hours Burnside's forces were exposed, the Union still had fewer casualties and Lee still had to retreat.
The tactics used on both sides were just horrendously inappropriate in a number of battles, that's a good bit of the reason for high casualty rates. I loved this description: "
In general terms, casualties of Civil War battles included 20% dead and 80% wounded."
It's strange to me that most of these generals -- even the most successful ones on both sides, but particularly the spectacularly unsuccessful ones -- never quite learned that firepower was the key *all the time*. During the early weeks of Grant's advance into Virginia, Lee must have used a trainload of ammo (and received at least as much back), and the bloodshed was incredible, but battle continued at a lower level (still intense) for months, leading to the siege lines at Peterborough and a war of attrition.