This is how they are a form of competition, lets imagine two retail chains. Marks & Spencer and Primark, both sell mostly their own items and do not share a catalogue even if they sell the same kind of items. They are however in competition with each other, same between Origin, Uplay and Steam.
They sell the same kind of items but not necessarily the same exact items, assuming a limited amount of funds (which they almost always are to some degree or another) that leaves the consumer with the choice of which game to buy and the secondary choice of shopping around to get the best deal.
Whenever that choice comes down too something like Dragon Age: Inquisition vs Assasins Creed: Unity and something off Steam they are in competition.
They sell the same kind of items but not necessarily the same exact items, assuming a limited amount of funds (which they almost always are to some degree or another) that leaves the consumer with the choice of which game to buy and the secondary choice of shopping around to get the best deal.
Whenever that choice comes down too something like Dragon Age: Inquisition vs Assasins Creed: Unity and something off Steam they are in competition.