Question

why should reactants and products have the same mass

Answer 1

Let's say 1 and 2 is the reactants. You add them to get the product which is 3. It has to be exactly the same. What goes in must come out.
\[1+2=3~makes~sense.\]
\[1+2\neq3~would~not~make~sense.\]

Answer 2

@perlamorales10 @~ @confluxepic @TheSmartOne

Answer 3

Conservation of mass

Answer 4

Yes. Exactly what @unklerhaukus said.