The duty is limited rather than absent
A landowner generally owes a trespassing adult very little: broadly, not to injure them deliberately or through reckless disregard. That is a meaningful protection and it is not immunity, and the moment something on the property is genuinely dangerous the analysis starts to change.
Children are the exception that matters
Where a hazard is likely to attract children who cannot appreciate it, the attractive nuisance principle can impose a real duty even though they were trespassing. Old machinery, open structures, water and anything climbable are the usual examples, and they are common on rural lots.
Defending a claim costs money regardless
Even where the law is on your side, somebody has to argue it. Liability cover pays for that defense as well as any judgment, which is why the legal protection and the insurance do different jobs and a landowner wants both.