When you push shit out, you're compressing that bottom section of your rectum to push shit out. (This is something you can feel for yourself if you want to stick a finger or two up your butt) You can't do the same motion with the top of your rectum, so you can't push things out of it. The inner band is hard to manipulate with external tools/your fingers. Like sure you can reach in there and stretch it a bit, but far enough for a large object to drop out isn't going to happen. If the object is too large and your inner band too tight, it's going to be extremely difficult to get out. Shit is going to back up behind it instead of pushing it out.

If you're wondering how I know all this, I've spent a lot of time with stuff up my butt.

You are right for small objects though, when I was going through puberty and exploring I used marbles in the shower, thought I had them all out, and then had to explain to my sister how I must've accidentally dropped that marble in the toilet.

Things sometimes don't come out due to shape or they lodge at a weird angle. This increases risk of bowel or rectal perforation as the body tries to push against an obstruction. Think opening a door but a desk is up against it. The cup can also break so now you'll have ouchie shards that can cut the bowel and create perforations or spill stool into the abdomen. Oh, and cause bleeding.