Most patent litigation is done on a fee for services basis.
Lawyers make lots of mistakes. While many of these mistakes are correctable, others are more serious and are not. Many lawyers are incompetent. We know of cases where lawyers have led people on, telling them in the beginning that we can file a lawsuit for only a few thousand dollars and that will scare the other side into settling, thus dragging the inventor into a black hole costing hundreds of thousands of dollars in litigation expenses. Remember litigation is in business, like used car sales.