Level 4 (e.g. highways in good weather) is something that people buying cars will pay a significant amount for as an option in good weather. It does nothing for those who want robotaxis. But the former don't care about the latter.

You have multiple groups of drivers including those who appreciate self-driving assistance on long boring highway stretches and may not even drive in cities that much and those who never want to get behind a wheel or perhaps ever own a car.

Level 4 doesn't necessarily mean just highways - it could also easily include suburbs with well-designed roads that have been mapped in excruciating detail.

You could force the car to avoid certain intersections with lots of close calls, or give the car advance info about unexpected objects detected in the road by cars up ahead, etc. which would make the task a little easier.

A selection of highways in good weather is just one example that seems particularly easy (relatively speaking), well-defined, and genuinely useful.

Current California regulations for remote operators are identical regardless of whether the car is considered SAE level 4 or 5. I'm not sure what constitutes "continuous monitoring" there (I could interpret that as anything between eyes on road at all times down to positive SOH messages arrived).

Apart from regulations, level 4 doesn't require a remote operator monitoring at all times. It just requires them to be available when the car recognizes it can't handle the situation. Depending on how frequently interventions are needed, that could mean a remote operator for every car, or one for every 10 thousand.

The important part about Level 4 is that the car recognizes its own limits and doesn't act outside of them. How practical those limits are is completely undefined by SAE. Level 4 is a continuum whose limit is level 5. We will definitely be seeing practical commercial value as we approach level 5 but haven't yet gotten there.