It really is silly not to allow gun noise reducers. What were they thinking when they wrote these old laws?
No one knows for sure. The legislative record does not give any reason for the inclusion of silencers.
http://gunwatch.blogspot.com/2016/05/review-criminal-use-of-firearm-silencers.html
My best guess is the Franklin Roosevelt administration were just grabbing as much power as they thought they could get.
The real push was to outlaw pistols. That was taken out of the legislation, and what was left made little sense.