Both Google and especially Apple have also spent millions of resources and money, making sure they are the only app store in use (by default).
They could have spent some of those millions creating a set of rules for independent app marketplaces to be able to distribute content on their platform instead of locking everyone in to theirs'.
The whole problem with monopolies is that they actually harm the consumer by protecting their monopoly from encroachment by someone else. So yes people really think decisions are being made by a monopoly to protect their monopoly (and their profits). A chunk of those billions they have is actually money generated by these monopolistic practices.
It's juvenile to imagine them sitting at board meeting and discussing "How can we protect our monopoly?", that's like saying the NSA is discussing by board meetings "how can we further invade everyones privacy?". But they most definitely are discussing how they can protect their market share and how to edge out competition.
How is this any different?
Google and Apple spent Millions 'claiming' Patents/IP, monopolizing new discoveries, and 'earning' Billions.
"Place Silicon Valley in its proper historical context and you see that, despite its mythology, it’s far from unique. Rather, it fits into a pattern of rapid technological change which has shaped recent centuries. In this case, advances in information technology have unleashed a wave of new capabilities. Just as the internal combustion engine and the growth of the railroads created Rockefeller, and the telecommunications boom created AT&T, this breakthrough enabled a few well-placed corporations to reap the rewards. By capitalising on network effects, early mover advantage, and near-zero marginal costs of production, they have positioned themselves as gateways to information, giving them the power to extract rent from every transaction.
Undergirding this state of affairs is a set of intellectual property rights explicitly designed to favour corporations. This system — the flip side of globalisation — is propagated by various trade agreements and global institutions at the behest of the nation states who benefit from it the most. It’s no accident that Silicon Valley is a uniquely American phenomenon; not only does it owe its success to the United States’ exceptionally high defence spending — the source of its research funding and foundational technological breakthroughs — that very military might is itself what implicitly secures the intellectual property regime." [1]
Isn't that what a duopoly is, by definition?