The most obvious reason is that using a musket requires both hands to load and fire, so a shield would be very cumbersome. They could potentially use a free-standing pavaise, in the manner used by earlier crossbowmen, but that would limit their freedom of movement and still be impractical if you wanted multiple ranks firing.
There was a lot more movement on an 18th Century battlefield than you might think from simple descriptions of opposing lines engaging. The effective range of a musket was a hundred yards or less. As the battle went on, smoke from the guns reduced visibility to even less than that. So units would have to advance to within a relatively short distance of the enemy to engage. They would also need to re-position to outflank the enemy or avoid being outflanked themselves. Then there's always the possibility of cavalry attack where the infantry would quickly have to form squares. Having to shoulder your musket to pick up and move your heavy shield with each maneuver would quickly become tiring and, I suspect, that the shields would get left behind as the battle progressed.
And, of course, there's the additional cost of supplying the troops with shields - it was probably cheaper to recruit replacement troops than it was to supply the original ones with practical shields. The same reasoning could also be applied to supplying the troops with breastplates. While it was possible at that time to make a steel breastplate that could stop a lead bullet, the resulting armor was heavy and expensive.