> But is anticompetitive behavior in itself illegal?
Yes. See https://www.ftc.gov/enforcement/anticompetitive-practices
> If so, why is it only being investigated now, 10+ years later?
By it's nature and the nature of our system it's often hard to prove or hard to actually prosecute, as those that engage in it are often large and powerful.
We have laws regarding monopolies not because they're "monopolies", but because it's a specific subset of anticompetitive behavior that was seen that the legislative body wanted to make it easier to prosecute, and easier to prevent before it actually happens (which is why the government rejects some mergers as bad for consumers).
That we didn't prosecute this before likely has many causes. From the token appearance of competition between iOS and Android to an unwillingness of politicians to go after companies that are seen favorably in the public eye and/or that have lots of money and contribute a lot of funds to reelection campaigns. Until a sizable chunk of the public is behind them, few politicians have the desire to be the first to call for reform that would be a major impact to some of the largest companies in the world, for multiple obvious reasons ranging from funded competitors to worried stockholders to how many people that company employs and if it will affect that.
Edit: Also, there's not usually a rush to investigate small or medium size businesses, or markets that are so new that it's plausible that competitors haven't shown up yet. That's the other half of the "Android and iOS aren't really competing" point. Initially it looked like competitors were around, but it wasn't always obvious that certain things just weren't on the take as competition until it became impossible to ignore. That everyone charged 30% and then all of a sudden there was massive change just as legislation was being considered is a dead giveaway, but also not something those companies could afford to not try in appeasement.