Why do junior engineers have to mostly write boilerplate code?
Junior devs lack experience, not intelligence. It's fine to give them difficult problems, as long as they're supervised.
I've worked with brilliant junior devs, sure, the code they wrote wasn't terribly idiomatic or maintainable, there were style issues, typical gotchas a more experienced programmer would be aware of etc., but it's not like they were fundamentally unable to solve a hard problem.
Junior devs lack experience, not intelligence. It's fine to give them difficult problems, as long as they're supervised.
I've worked with brilliant junior devs, sure, the code they wrote wasn't terribly idiomatic or maintainable, there were style issues, typical gotchas a more experienced programmer would be aware of etc., but it's not like they were fundamentally unable to solve a hard problem.