That is because we've built tools that are good enough for most use cases, and the cases don't actually differ all that much. For every new case however, new software has to be made. It isn't getting any easier, there is just more of it now.

The problem is that a "problem" is essentially a fractal; it needs a defined accuracy and scope to be solvable. Differing scopes and accuracies again require overhauls of software that would otherwise be acceptable for the same task.