Answer:
Expert witnesses are exactly that, experts in their field. Most of them have a degree, one or more, or higher education degrees and have been working in their fields for years on end thus whatever they testify to is hard to impeach in open court. They don't acquire all that expertise overnight and are no slouches when it comes to fighting (with words) with other expert witnesses on the same field.
They are not cheap and some of them can charge in upwards of $500 if not more per appearance in court.