Why are pressure sores so dangerous? I mean, it’s just a sore, right?

Pressure ulcers. We know that patients who develop pressure ulcers are often 'at risk'. The skin is the largest organ of the body and sores can be a sign of organ failure in general for the patient. Also, a pressure ulcer can become very deep and affect deeper structures causing, for example, infection of the bone that will require extensive treatment. Rarely, patients develop serious, life-threatening infections.
No. You obviously have not seen the range and size, this forum is not appropriate to answering this type of question.
Not "just" a sore. Pressure sores are particularly difficult and "dangerous, " because of why and how they got there in the first place. A patient who develops a pressure sore is either unable to reposition or unable to feel the sore developing. Since the sores develop in an area of constant pressure, changing the pressure points is the only way to effectively heal the wound and that can be very tricky.