Could you provide some examples? I too would like to understand how a company can find it hard to find out which of their patents, for example, is being infringed -- and still have a legitimate case.
There may be a number of ways to make a product, but only one of the ways of making it is infringing. It's not like companies go around telling people the secrets of their manufacturing plant. Many manufacturing plants, especially in pharma and also in tech, have high security.
And even if you could find out (let's say by corporate espionage) - that evidence may not always be admissible (or reliable) in court.
The opposite is the famous example of lawyers from IBM basically extorting money saying "do you want us to go back to Armonk and find some patents which you DO infringe?" The implication is "pay us now and we won't make life harder for you using the legal system, because we think we can". That is not justice, nor is it in anybody's interest except the holders of many patents.
The fact is that this extreme example is just an example. Lots of companies just send a bunch of intimidating letters to people and hope it sticks - because it's free!