For the same reason, it's impossible for NIH to internally possess all of the expertise to competently review every proposed project. The solution to this is Study Sections. NIH invites scientists (generally people who are themselves funded NIH extramural researchers) to participate in a Study Section, which reviews grant applications responding to a specific funding announcement (NIH-identified research priority). Each reviewer assigns each application a score, and the top-scored applications are then considered for funding.
It is not a perfect system, and I think every NIH-funded researcher has some complaints about how it works. But it's the system we have, and on the whole the past 50+ years of biomedical research in the US show that it basically works pretty well.
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