Thank you Kuba! This makes sense.
I noticed that if I used "/" instead of "Control+/" it works but does not convert to traditional form in either the input or output. Is there a way to fix this so it converts? The code below coverts to traditional form... but does not use boxes. Is there a way to convert with boxes?
Manipulate[Row[{a, "+", b, "=", Simplify[a + b]}],
Row[{Control[{{a, 1/2, ""}}], Control[{{b, 1/2, ""}}]}]]