Lazerface
Eagle Member
Most top events it isn't the policy any longer, as others have pointed out regarding live scoring becoming more ubiquitous.
At other events (B tier and below, typically), the biggest reason there is often just one scorecard per group is a cost-saving measure. 36 scorecards for a 2-round, one course tournament is easier than 72 or 90 (X 2 if using fresh cards each round).
Between phone apps and eraseable/re-useable scorecards becoming more common, it is really simple for any player to keep score for themselves or their whole group in addition to the official scorecard(s). It's a good practice for every player to get into, not just for purposes of preventing cheating, but also it can be good just to have a personal record of one's rounds.
Even beyond that, the easiest way to prevent shenanigans with the scorecard after it has been counted and verified by the group is for the whole group to turn it in together. Not just to prevent any one from pencil whipping, but also to make sure no one walks off with it accidentally and forgets to turn it in...something that, in my experience, is more common than pencil whipping.
To add to that, I think myself and many other players know exactly what my card mates are scoring throughout the round and where we all stand at the end. I totally understand the mentality of not "playing your own game" and not paying attention to scores, but as crazy as it sounds I feel like I can go through a full round of 4 players in my head and reply everyone's round hole by hole and if at the end the final score did not add up to what was in my head I could go through the card and find the culprit. I'm no savant and feel like there are many players like me.