They haven't shown it yet because they aren't ready to show it yet. If they wanted people to see it, they'd show it. Maybe they think that youtube videos or trailers won't cut it, maybe they're waiting for the game to be in a more finished, more polished state
That's exactly the problem and the reason people are concerned. The game has less than a month to go before it goes gold, and there isn't so much as a screenshot of two of the four games in the package. The media we have seen isn't exactly inspiring, either. Halo 2 Anniversary campaign is running at sub-1080p with framerate problems in every video they've approved for media release. Halo 2 Anniversary multiplayer has been buggy and crashing at publicly demoed events.
There's also very little detail about, well, anything. Playlists, matchmaking, netcode, graphics and performance, etc. It's a pretty stark contrast to Halo games of past (particularly Halo 2 and 3) where the community was made to feel really involved in the development process through betas, regular updates, screenshots, footage, details, etc.
This franchise is one that's near and dear to many people's hearts, myself included, so of course some of us are concerned. It's not a matter of "doom and gloom" or "whining" or "driving up post count" or whatever nonsense you want to label it. It's legitimate concern due to a lack of media and information on a game that's less than a month from going gold and has been very tight-lipped on information in contrast to previous Halo games.
The Master Chief Collection represents a possible rebirth for the franchise and an absolutely huge opportunity for Halo as we know it to establish its foothold this generation. Our concern is out of love, not out of a desire to see it fail.