I don't know about the US but in Canada a "Justice of the Peace" has a full-time job to signs police warrants not regular judges who deal with criminal cases. I've hear that Justices rarely deny police requests and they deal with police all day... there is never a defense attorney present when wiretaps are agreed upon. Oversight is always well after-the-fact.

They are only ever question way later into the legal process (sometimes over a year afterwards) and always with a different judge and circumstances.

Law enforcement have a far easier job getting it approved than it is for defendants to question it's legitamcy. And even if it is ruled illegal the wiretap still happened and the defendants privacy was still unduly violated. But there is almost never any repercussions outside of the potentially losing a legal case against a defendant.

The chains of responsibility between where decision-making takes place and scrutiny is made are very disconnected.