" This is the 21st century, we’re all connected, so why wouldn’t you do this?"
Because in order for me to give you money, I demand a product that is already of a high quality, not an unfinished product that I'm paying to help beta test. You're publishing a game on the Xbox Live marketplace, not Kickstarter.com.
" They like to trot out dlc right away these days... "
Like Jeff said on one of the Bombcasts. The game has been done for months and there has probably been some people working on the game that have been done for even longer than that. Those people aren't going to sit around and do nothing, they probably started working on the DLC right when their job was done in the initial game. "
Quoting for emphasis. This is the exact reason why you're seeing tons of day 1 DLC.