There's an actual good reason for this, which is that items are way stronger in DOTA2 and a single item at the right time can change the course of the game. It's up to you and your team to keep tabs on all enemy heroes so that you aren't caught off guard by, say, Storm with Orchid, or SK with Blink, or Void with Refresher.
At higher levels, whenever someone gets one of these key items, it's always called out before they get the chance to surprise you with it. As for the suggestion, they could do it, but there's no real reason to. In the end, it comes down to how much Icefraud wants to feed information to the players versus working for it. It's Rito's design philosophy to focus on the tug-of-war between Champions as much as possible, but this isn't really Ice's focus at all.