To be honest I think this comes from a lack of experience on their end. They might be technically skilled but haven't really experienced trading off complexity for performance/memory use/"elegance", etc., or weren't the ones to suffer the consequences of an overcomplicated system.
I think the more experience you have the more you appreciate that a clear, maintainable and "pretty good" system always wins in the long run over a solution that's better in some dimension but opaque and unmaintainable, unless that dimension is really business critical.
It also sounds like they're just bored to be honest, and want to implement something because it's interesting or challenging, not because it actually makes sense. Maybe the best thing is to find them an interesting project that's actually useful.
I think the more experience you have the more you appreciate that a clear, maintainable and "pretty good" system always wins in the long run over a solution that's better in some dimension but opaque and unmaintainable, unless that dimension is really business critical.
It also sounds like they're just bored to be honest, and want to implement something because it's interesting or challenging, not because it actually makes sense. Maybe the best thing is to find them an interesting project that's actually useful.