Why hasn't anyone asked Trump about witness tampering or being accessory to a crime?

The media and many do consider what Trump did with Cohen as witness tampering, but if he does know of crimes that Cohens family has done then he is still an accessory after the fact. Now whether they are guilty or not guilty of some crime, he is still guilty of threatening Cohen by going after his family.

I wonder why no one has asked him what crimes is he aware of? And if he did know of these crimes, why did he wait to till now to report it?
If he says he has no first hand knowledge but heard rumors, then it makes him more guilty of witness tampering and trying to use the power of the presidency to put pressure on the witness.

If he does know of crimes then they can add one more thing. Accessory to a Crime. Why is this not even a thing?