That's an odd one. I suspect it was done due the game engine having no provision for a leader above the level of Army Commander. The game ranks Grant as an Army Commander but with no army under his command and, as Blake says, "Grant is a supernumerary". Additionally, the game fixes Grant in a position near the Rapidan. Grant's actual headquarters was far closer to the action (probably near hex {75,68}).
Although Grant's memoirs have him issuing orders to Meade on a few occasions a check of the Official Records reveals something different. The total of correspondence from Grant to Meade during the battle - one. Here it is (
https://babel.hathitrust.org/cgi/pt?id=coo.31924077728289&view=1up&seq=617&q1=). Even that 'order' reads to me more like suggestions rather than specifically directing movements of the forces during the battle. The order frequently says things like: should; could; if; and may.
Here's another complication for that battle. At the very least Burnside should be allocated under Grant's command. As Grant says, "Burnside at this time was not under Meade's command, and was his senior in rank." Also, Grant says:
"Burnside, who was coming up with two divisions, was directed to get in between Warren and Wadsworth, and attack as soon as he could get in position to do so."; and
"Burnside had three divisions, but one of them—a colored division—was sent to guard the wagon train, and he did not see it again until July."
The game, and other OoBs, show four divisions under Burnside (including the "colored division"). Grant also seems very impressed with the Army of the Potomac's wagon-train. He devotes a substantial paragraph in describing it and how it operated saying it was "a wagon-train that would have extended from the Rapidan to Richmond". Keen to protect it, it was probably no surprise that he allocated a division to guard it.
Still, Burnside's Corps didn't really do anything during the battle so it's probably correct for the scenario to have them 'arrive' and be fixed in a position out of the way. [Grant says Burnside was delayed moving beyond Wilderness Tavern due to the "difficulty of making a way through the dense forests".]