Enough people are willing to change it that just paying to be the default wasn't all that effective for Bing or Yahoo. Microsoft's previous strategy was to pay to make Bing the default and remove people's ability to change it. In particular, they took advantage of the mobile phone provider oligopoly in the US and other countries to pay off all the major providers to do this, so that consumers didn't really have a choice to not have their devices send searches to Bing. Google ruined that strategy by outbidding them without even imposing the same kind of lock-in that Microsoft did, and so Microsoft are trying to get the US government to ban them from bidding.
Sure, let’s change it to “it is trivial for Microsoft to spend its greater than Google’s profit to buy the default search position on Apple devices.”
So they can other have a just as good as Google’s product that they do not want to spend enough on to make default, or they do not offer enough of a value proposition for people to go to settings -> Safari -> default search -> bing.