Because every other species of rabbit is solitary in nature. Where they can, the European wild rabbit (the domestic rabbits' ancestors) tends to spread out; but when good habitat is limited, they can live in (rather uneasy) groups. There are no friends in wild rabbit colonies; there are dominant animals, and subordinates that spend a lot of energy just staying out of the way of the dominants. This kind of aggressiveness is normal for rabbits; a lot of people have seen domestic rabbits shred each other (even siblings and mother/daughter combinations) when there isn't plenty of room to get away from each other. Baby rabbits instinctively seek each other's company; their survival depends on it. This tendency is often lost when they become adults, as a need to establish their own territory asserts itself. A person who is breeding meat rabbits is often much more interested in a doe's mothering instincts and productivity rather than her friendliness; a truly alpha doe that would eat the face off another rabbit isn't a problem when housed alone. Likewise, a timid doe won't have to waste energy running away from other, more dominant animals, but can raise her family with a minimum of stress. As long as what you are dealing with are rabbits with more laid-back temperaments, you can keep them together, but even the House Rabbit Society has had a few that they had to house alone.