Not sure what type of "greenfield" startup experience you've had, but most of the work I'm talking about involves solving problems that most people simply don't have the combined skill set to solve. Typically problems that involve a substantial amount of quantitative skills combined with the ability to bring those solutions to prod.
Nearly all of the teams I've joined had problems they didn't know how to solve and often had no previously established solution. My last gig involved exploring some niche research problems in LLM space and leveraging the results to get our first round of funding closed, this involved learning new code bases, understanding the research papers, and communicating the findings publicly in an engaging way (all to typical startup style deadlines).
I agree with your remarks around "greenfield" if it just involves setting up a CRUD webapp, but there is a wide space of genuinely tricky problems to solve out there. I recall a maintainer style coworker of mine, who describe himself similar to what you are describing, telling me he was terrified of the type of work I had to do because when you started you didn't even know if there was a solution.
I have equal respect for people such as myself and for people that you describe, but I wouldn't say it is more challenging, just a different kind of challenge. And I do find the claim "you don't learn the actually valuable lessons" to be wildly against my experience. I would say most of my deep mathematical knowledge comes from having to really learn it to solve these problems, and more often than not I've had to pick up on an adjacent, but entirely different field to get things done.