It is not true that your oven is at 400 degrees, that's a convenient fiction which is easy to put on a dial.
In fact, an oven has two heating elements, which are pumping energy into the inside of the oven. The oven dial just determines the energy that will be emitted, averaged over a longish period of time. The energy is transferred to the oven walls, the air in the oven, the food, and the pan material.
Out of those, the food is the worst heat sink you have. The air has almost no mass, and is heated quickly. The metal conducts heat well, heats up quickly, and starts emitting itself once heated. The organic matter in your food is slow to heat up, especially by conduction from the warm air. Direct radiation from the heat element contributes a lot, especially to surface browning. And of course, for the conduction to happen, you need the air layer close to the food to be warm, not a wad of cooled down air which just exchanged its energy with the food.
As soon as you fill your oven up, these effects are disturbed. For conduction, you get the same amount of energy, but it has to be exchanged with a much higher mass, so you get less energy per unit of food. For convection, Ecnerwal already explained that the foods block the flow of air, so the air surrounding the food is colder than it would have been. And for radiation, the upper crust of the uppermost food absorbs everything coming from the upper element, and the lower crust of the lowermost food absorbs from below.
The oven is not a completely dumb device, it does have a sensor, and so it should keep the heating elements turned on for longer in such a situation than in an emptier oven. But first, as Ecnerwal pointed out, the sensor has an arbitrary position that is not so close to the food, and you have created a situation in which the heat distribution within the oven is very unequal. And second, I would guess that, even if it goes on for longer, it is just not sufficient to cook your food quicker. This is especially true with modern baking techniques, where we use light ovens and barely preheat them, so that they are nowhere close to thermal equilibrium when we add our food. This is a really good thing for both our wallets and the environment, but it does require you to get used to cooking times that vary with oven loading.