"It really makes you wonder - why didn't the allies make some of those "significant troop movements," maybe catch the French in the act of crossing the Sambre, and wipe them out piecemeal?"
Essentially, this is asking why didn't Wellington do something bold and offensive like what Napoleon might have done in the same situation. The answer is that Wellington wasn't Napoleon. Wellington was famous for his defensive style of operations with a strong emphasis on sacrificing as few of his men as possible; something, of course, Napoleon never had much concern with.
While Wellington may have been surprised exactly when Napoleon attacked; he certainly wasn't surprised that the attack came and, it seems from the result, that he was reasonably well prepared for it.