"Silencers", or as they are more properly known, "suppressors", *are* legal now. However, they require an expensive Federal tax stamp, and the paperwork required takes 9-18 months to process. They also require a special federal license to manufacture or repair. And you technically commit a felony if you hand a firearm with a suppressor on it to another person to use at the range. Or drive across state lines with one without filing the proper paperwork.
As a result, a suppressor tends to be expensive - ~$500 or more. And can't be maintained legally by the user, even though innards of many common types are consumable items.
Suppressors are common firearms safety equipment in Europe. They protect the hearing of the user and other people nearby. Some nations *require* their use when shooting. You can buy them over the counter there, and they cost about $25.
I teach a lot of people to shoot, especially younger people. Hearing protection is a big deal. I would love to be able to use suppressors in class, as you can in the more enlightened nations of Europe.
I'm hoping H.R.367 - the Hearing Protection Act of 2017 - will pass this year, if Trump manages to stay in office long enough to sign it.