Let's say that 1 in 1,000 people in an army are wizards. How many of those are high enough level to cast spells like fireball? Is it 1 in 2? Or is it 1 in 10? Because if it's that latter you now have only 1 wizard per 10,000 troops. You aren't going to even notice them.
But let's say they can all cast fireball. Assuming you don't use the alternate long rest rules like I do, the odds are you're still only going to have 2-3 fireballs you can cast per day.
Furthermore let's be generous and say the caster is close enough to the front lines to get full effectiveness. I'm not sure how often that would really happen, but we're talking best case or close to it.
According to the DMG TOTM rules you'd only get 4 targets so I ignored that and instead counted out on a hex grid to get 30 assuming tight formation. So every day in the best of circumstances a wizard could fireball 30-90 targets.
That's a drop in the bucket and assumes the wizard is in the right place at the right time. Remember, that wizard is not going to be able to contribute much else to the fight.
At the same time, a wizard will be a prime target for the enemy. So while they would have an effect, it would likely be less than that of other units such as heavy cavalry.