Because Calgary doesn't want that return. It's a 2way street in trades.

Calgary has said all along that they want a player of similar age for Bennett.

He's definitely better than Marleau was at that time but that cap is steep for a third line winger/center on an already stacked Pens offense. And they've been good even without Gino for like 10 games or however long he's been out. The moment I saw it I instantly thought boy's club as well.

1. If you are comparing the two the major things you get with Carter over Bennett is shot volume and goal production. Carter has 25 goals since the start of the 2019-2020 season. Bennett has 12. Carter has 21 assists to Bennett's 12. Carter has 299 shots to Bennett's 149.

2. Carter can still skate, is hard on the puck, and is a high volume shooter. If you look at his goals this season that will be clear. https://www.youtube.com/watch?v=TZZPKtpchjo

a 3rd round pick?
That's not really expensive.
It only becomes a 2nd if they make it to the cup. If they do that and he plays more than 50% of games than he probably played a role in it so who cares.

The 4th is the cost of the recapture. If he retires, which is probably likely, the penguins get about 400k in cap space. Which explains the pick.
This is the kind of move I'm surprised TOR didn't do.

Carter has 7 5v5 goals this year. They hope he can produce more on a better team.
I'm not saying he will, nor am I saying I think it's a great deal but it's not a horrible deal. In the end he has to play better than Gaudreau to be a center on this team. Unless they are playing him at RW.

I like this trade, it brings leadership and experience into the playoffs.

There's a lot of things that you can knock about Carter's game since his injury, but his skating is not one of them. He's quite fast and is one of the best skaters on the Kings.