According to the Hunting Rifle (Fallout 4) page on the Fallout wiki, the bipod is cut content. There's no way to obtain it in-game. If your playing it on PC however, it can be obtained through console commands.
Because some dev was working on it and threw it into the main file location so someone else could have worked on it. Chances are that it needed some type of prone animation that was supposed to go with sniper rifles, but they couldn't get the prone animation to work properly. If the animation doesn't work, then there is no point in putting in a mod that works on a broken mechanic.
Can't find the console command, but http://www.nexusmods.com/fallout4/mods/3059/? is a mod that does the same thing.
That bugged me for a long time too, It just looks aesthetically pleasing.... and of course by showing me a picture of something I can't get in the game simply makes me want it all the more. I can't have that? but you showed it to me.. Don't show me things I can't have.
Here's the link. My guess is that what starkoas said is true. A prone animation was in the works and it wouldn't work properly, so the bipod mod was cut.
Now keep in mind that the Cryolator was supposed to be in Fallout 3, yet because that weapon wouldn't work (it crashed the game) everything associated with the weapon was cut from the game(it was meant to be a craftable weapon like the Shishkebab). The only item that was left in Fallout 3 that was part of the Cryolator's crafting schematic was the crutch. The crutch was used in the crafting of the Railway Rifle in Fallout 3.
Link below:
http://fallout.wikia.com/wiki/Hunting_rifle_%28Fallout_4%29