Why is it not recommended to underline file names？

[ixuer]

It's hard to see what a file name means without underlines. I've read source code from other good frameworks where the filename was underlined, even if it’s directories may not be separated with underlines.

[kslamph]

it's a convention. so follow the convention.
if the question is why there is such a convention.
that's because _ has been assigned to some special meaning in golang.
to name a few:
suffixed _darwin will be build only when os is darwin
suffixed _arm64 will be build only for architect arm64
files prefixed with _ will be ignored by go tools;
go test will run suffixed _test go files;

so _ is a "reserved symbol in filename".