While place bet on sports betting especially football I don't think of placing bet on cards as I think cards are more difficult technical to win than just normal win, over, under, home or away, home win or draw and away win or draw. Also like how many goals in first half and how many goals in second half, both half total goals. These options makes it more easier than having to bet on either yellow or red card.
The betting sites still make use of the odds to have the advantages. This can still make some people to bet on card because they know no selection is guaranteed. Betting on cards is not common but I have seen people that are betting in it. If most people are not betting on it, the bookmarkers would have removed it from what bettors can select while betting.
What I am just curious to know are people that will use referees to bet on cards. I still doubt this. According to many PO on this thread, they also have my own opinion.