Actually the question is not quite right as catalysts will often take part in a reaction.
One of the ways catalysts can work is that they will react with the chemical reactants to give intermediary products. These intermediary products are unstable and will then react again, or decompose, to produce the final products leaving the catalyst in its original form.
Simplistically, imagine A and B reacting in the presence of catalyst C to produce product AB
Stage 1 might be
Stage 2 might be
This leaves the catalyst unchanged and in its original state at the end of the process.
Hence although catalysts are not usually consumed in the reaction, they can actively take part (catalysts can be involved by other means too).
I only came across this at University. At school, the discussion of catalysts is normally limited to the fact they are not consumed, they speed up a reaction and they lower the activation energy.