I think you've accurately identified a real tension here, and this connects with a fundamental critique of EA as a movement, which is that it is too often focused on measurable outcomes rather than systemic change. I tend to agree that this critique has teeth and applies to the way EA is often practiced.

I do want to highlight that Global Health work is not inherently a temporary fix. Global Health work frequently can (and should) focus on improving existing health systems, not just having a temporary impact. By addressing the root cause, you can make a more permanent difference (and be more cost-effective while you're at it)

So why are more EAs focused on Global Health instead of Global Development relative to your expectations? In my opinion, two major reasons are

1. Some people are likely overly focused on measurable outcomes over systemic change.
2. Some types of Global Health work is more systemic than you give it credit for.

talk to @David Nas and @Karthik Tadepalli ha. There's increasing work within EA on development directly There are big questions around how tractable it is, how much EA influence can actually move the needle with huge money injectors active like the imf and world back, to and market forces as well. 

And yeah like @Evan LaForge said to some extent development needs good health and education to happen (a bit of chicken and egg)