What’s the point of a seal?

My point is, regardless if you think they don't care or not, you treat it like it's supposed to be treated. To protect yourself. As many have pointed out here, make them inspect it. Take a picture of it with surroundings, because if you don't, it's your word against theirs and you have no proof, in the case of claim...that may get you terminated. Understand that concept? Don't worry about what other's do. Do your job as a professional, respecting the purpose and CYA (Cover Your A sets). As for a reason they don't inspect? Maybe it's to file a claim against you or your company for shortages real or imagined.
I once (argh when pulling a van) did a live load, and when I got the paperwork I put SLSC (Shipper Load/Shipper Count) on the BOL. The shipping punk told me I can't put that on the bill. I told him if I can't put the facts on the bill, then he can take it off so I can count it. He was puzzled. Because I have been shipping/receiving in a past position and I know that it matters that if I didn't count it, it's not on me. Period. Does that help you understand that they might have a scam going that they can get money from you or your company by filing false claims you can't defend?

 

Did the guard walk around your trailer? Approach from the back? If not, he didn't do his job. Again, don't worry if others don't do their job. Do yours, and those situations you mention are the very ones your 'spidey sense' should tingle and tell you to CYA, by taking that picture/video/insisting that the seal be checked....that's what a professional would do, covering his company's butt and his own. You are right, however, this is a question you should be putting to the people that aren't checking and see what their answer is. I would, but then again, I'm not shy and reserved.