My answer is pretty basic. It is just easier and more practical for both parties to settle than for either side have a protracted battle. It is the rare bird who will battle it out of principle.
Back in 2008 with my squabble with Getty Images, I seriously considered settling because they offered $500 to settle. But I was pretty pissed about the situation and the circumstances surrounding it because I felt totally blindsided by ignorant actions taken by an overseas graphic artist who got me in hot water.
I kept doing research and the more I dug into Google, the more pissed I got and what I fell into which is why I started the ELI website, which in turn, attracted more information and insights.
I thought to myself, fine, I might lose but I was not just going to hand over the money. I would muddle my way through the federal court system and fight tooth and nail the best I could. I figured I would just stumble my way through the federal court system and probably lose but I would learn a shitload from the entire experience. I viewed it as "tuition" for a front row seat at a "live seminar experience". Without having to pay legal fees, I figured I had a lot more room to maneuver and wasn't as constrained.
I was emboldened when I found how much opposing lawyers HATED dealing with pro se defendants. As a backup measure, I won't go into specifics but let's say I was going to exercise my lawful First Amendment rights in a very big way because ELI has already been launched and I had a growing platform.
I figured if I was going to lose, I wouldn't go down without inflicting some scorched earth along the way. This is one of these situations where I tell people NOT to follow what I did. I was (and probably still am to a degree) a bit irrational. (Do as I say, don't do as I did which was fraught with unknown risk.) I was never quite sure why Getty stopped pursuing me after only 3 or 4 go-arounds. My case was surprisingly not escalated to anyone.
The point being is that one never really knows for sure why certain cases go forward to lawsuit and others don't go anywhere. But I have a good idea who are more likely targets than others and who are more likely to receive lawsuits than others. But my opinions are grounded in life experience/insight and personal/professional observations than any empirical evidence.
I tend to believe in executing a STRONG strategy to encourage a highly-motivated settlement. The downside to that is that MOST people don't have the knowledge or have the internal fortitude to do so.
There are lots of reasons that these court cases never go through a complete trial.
First, the outcomes are predictable. This predictability comes from the fact that 99% of photo infringement cases are straight forward. Typically, the only real question is the award amount. This predicabiliy greatly increases the chance for settlement as there is very little reason for one party to think the outcome will be wildly different than the other party. As long as one party is not drastically over reaching, they settle.
Second and equally important is the fact that almost all copyright cases involving photos can be resolved with a good motion for summary judgment. It is very rare that there are issues of fact that need to be determined at trial by a judge or jury in a photo copyright case.
Third, many judges require that the parties go to court sponsored mediation before trial. The mediators, who are either retired judges or experienced litigators, are very good at getting parties to settle the claims.