To stay with your examples, some of the Gate's foundation efforts have largely been tech efforts, some haven't. Some of the things I mentioned off-the-top have potential to reach 10s to 100s of millions of people. MFA is a big $ number partially because the US healthcare system is expensive and inefficient, so that draws focus on $ saved ... other efforts may focus on lives impacted at much lower $ impacts.
Most public policy work isn't as big as MFA either.
I'm not saying policy work isn't important or impactful. I'm rejecting the idea that you (generic 'you') can't have similar effect in technology to what you might be able to achieve in policy; especially the idea that this is so clearly true that the "right" thing to do is quit technology to go into policy. Even assuming you can be as effective in policy work as you are in tech work, which isn't a given.
With that said, here is my example: Medicare For All would save us $450 billion, and avoid 68k preventable deaths, every single year. I find it a stretch to suggest that ambitious goals such as that "lack imagination". Such impact is simply not obtainable with tech or NGOs alone. Even Bill Gates has needed roughly $50 billion to drive forward his foundation's humanitarian efforts (but admittedly, is very efficient, with estimates of having saved ~122 million lives through their work).