Closed playtests are usually with very in-development builds. People post the barely functioning game to social media and the game gets bad press. Release day rolls around and no one buys it because "that was that one game that looked bad a while ago"

This seems like a stupid train of thought but a lot of people think like this

Others have pointed out the concerns around negative reviews of things still subject to change, but the other aspect is just the relations with media.

I'm sure tons of journalists have been playing. And probably even working on content covering the game, but not publishing it yet. Once valve is ready for coverage they'll have polished content ready. And valve can control the timing so that coverage happens right when they want the hype like maybe a few days before an open beta.

By covering it early you encourage other journalists to do the same, rushing out low quality content to get the views before others do. And for valve to not let any journalists see the game early to avoid this.

The biggest thing would be that a game under playtest is likely to undergo drastic balance changes and potentially even changes to core gameplay, a review of a game in that early of a state would likely not reflect the finished product, and is unlikely to be updated or taken down when the game is released, this possibly poisoning public opinion with content that doesn't reflect the actual game.