Because any other agent would have been more useful than a chamber that can't shoot. Almost every pro had said this before. If you're having a bad day, there's nothing wrong with that. But try to at least pick things that are useful and bait for the team. He just had a blind confidence to pick the chamber at that exact moment and it turned out badly for him. (I'm sure it was the team's decision to put him on chamber. It's not saying it's he's at fault.)
Saying he should play Vyse is me being in hindsight. Because pushing Johnap onto Vyse while he played Chamber sounded good on paper, but it fell flat when Leaf was already having a bad game that day in the previous four maps. Kinda reminds me of what every pro had said. If you're having a bad day, let your teammate bait you.
I'm going to repeat myself, leaf picking the chamber in that exact moment was him showing confidence. That's why I said the confidence vanished after the game. NOT before the game. All of this is in hindsight, but there were cracks already showing in the previous four maps. He was clearly having a rough series.
A KJ turret and alarmbot would have been more useful in terms of map control. A sova with Tejo and him playing Vyse would have had more impact than a chamber. But hey, there's no need to be mad over spilled milk.