Because King George III wasn't so insane as to kill 750,000 people to subjugate the seceded states. He stopped at around 15,000 dead.
And what would have caused slavery to end by the late 1860s-early 1870s as has been claimed?
What does slavery have to do with the Rights of the Colonists to be free of the United Kingdom?
Make it good, because your answer will immediately be applied to the 1861 war.
In fact, US deaths totaled over 25,000 with another 25,000 wounded.
Those alone were equivalent to 500,000 in the US Civil War (3 million population in 1780 vs. 30 million in 1860).
But Brits and their allies in North America, including sailors, suffered at least that many dead & wounded over the 1775 to 1783 time period.
So, by Civil War equivalents, deaths on both sides in the Revolutionary War roughly matched to those of the Civil War.
But, of course, King George was in no way, shape or form directly counting up deaths as his decision-making factor on whether to stop or continue war against Americans.
One reason is that none of those involved were voters for the King's or Parliament jobs.
So, what really mattered to Georgie Boy's parliament was battlefield success vs money cost of operations.
When the money costs got too high and success too little, then they threw in the towel and called it quits.
By contrast, President Lincoln's job was on the line if his war efforts failed.
Fortunately for Lincoln, they didn't.