> Wouldn't the easy problems have been solved as they are by definition, easy?
Only in an efficient market. And real world markets often have vast inefficiencies. The reasons they're inefficient don't always generalize, either. Companies can behave in certain ways because of personalities (2024 Twitter vs 2018 Twitter), embedded institutional norms (2024 Boeing vs 2020 Boeing vs 1980 Boeing), changing conditions (any 2024 startup vs any 2019 startup's finances), or a million others that don't necessarily spell doom for trying a different approach.
One that's relevant a lot in the real world is market share. Large organizations very frequently get away without doing easy good things for long periods of time, which they can do because network effects and platform lock-in are powerful defenses against disruption. A lot of startups get founded on the idea "large incumbents are not doing this easy good thing, so let's do that and beat them". The fact that this ever works is a sign that incumbents must be leaving a _lot_ on the table, or their lock-in would never be overcome. The very existence of the startup scene is proof of the frequent inefficiency of markets in the short-to-medium-term (or at least of investors' belief in such).
Even in cases where the incentives _are_ aligned and the market _is_ efficient, the world is often in non-equilibrium states. I like to think of incentive gradients as something akin to a (very complex) differential equation, and consider what _simple_ DEs can teach us about them. Consider, say, Newton's law of cooling: dT/dt = -k(T-T_e). Some calc 101 will tell you that solutions to this equation trend (exponentially! so not even slowly!) to a constant stable equilibrium T = T_e. But if you try to use that analysis on a fresh batch of french fries, you're going to get burned, because it turns out T(0) is very relevant to predicting T(1 minute) for realistic values of k.