That's where the word "secret" comes in in the definition of Collude. Due to national security, unless a court actually declares collusion in a criminal case (such as the Monafort ones) it can't be prosecuted, which is why certain people in a certain building in a certain nation's capitol are trying to gut and discredit the legal system that could put them in prison for years. (One individual, of advanced years, could end up as such for life)
Collusion itself is not against the law. What is done during the act, however, is.
Using foreign agents to affect the outcome of another country's elections is illegal by our laws. Wire tapping and computer fraud are also illegal. Those will end up costing those involved their freedoms, their fortunes, and their reputations.