You don't earn the gravity gun until about halfway through the game, at which point the object placements change. Half-Life 2 also has a tiny throw distance, so getting explosive barrels placed correctly is hard when you've got 10+ goons firing automatic rifles at you. The base throw distance is low to emphasise the effect of the gravity gun.
The whole concept of those barrels being there is to keep reinforcing to the player "Yes, these are a good thing when used correctly", making the player aware of how they function and where they're best used. Notice how they're never used the same way? When you get the Gravity Gun, the game stops placing the barrels for you and you're expected to do it yourself; it creates a change in logic that builds player confidence and endorphin levels when they figure it out. Later in the game you then stop throwing barrels and start throwing other items for similar effects. It's a very specifically crafted logic tree.
Congratulations, you've played games most of your life and you know everything there is. Put a big pat on your back and forget off. Valve isn't making games only for the most experienced gamers; they're building experiences everyone can enjoy, and they have a very specific experience tree they want to build. Bioshock isn't exactly designed for new players; the whole premise of the game is to deconstruct FPS games.
Stop thinking of HL2 as a story and start thinking of it as a sandbox. You have a bunch of toys to make a bunch of set-pieces. What do you want the player to learn? What's the best way to teach them that?
