Except it doesn't, also because of psychology.

The ancient Egyptians learned this the hard way. Many of their tombs were marked with graphic warnings threatening anyone who disturbed them with various horrible ends. But most of them were robbed regardless, and those that weren't were eventually cracked open by 19th and 20th century archaeologists.

That wasn't because the people violating those tombs didn't understand the warnings (the archaeologists certainly did). It was for two reasons:

1. Some people, like the archaeologists, are going to read the warnings and dismiss them as dumb ancient superstitions; and

2. Other people, like ancient grave robbers, are going to read the warnings and think "Huh. They really want me to stay out of that place. There must be something really valuable in there!"

I wonder if Indiana-Jones-style death traps would successfully show trespassers that the site's builders were serious, or if this would only attract more curiosity.

Most of the best traps can only be sprung once, or at most a few times. So you get the first trespasser, but everyone after that just walks over it.

And honestly, Indiana Jones style traps would never survive 10 years of deferred maintenance, much less 10,000 years. Even simple traps like weak floors over spike filled pits would tend to rot and collapse over the years, especially if you need to build it lightly enough that a single person can trigger it unknowingly.

The only Egyptian tombs that survived to the modern era were the ones so well hidden that nobody had managed to stumble across them.

Also not having a pile of gold at the end of the tunnel might help.

Then again, we can't really know if the future civilization will consider out spent nuclear fuel as a pretty good store of value.

the radiation is the death trap

I was thinking the same. However, the problem is not to avoid people from getting in and causing harm to themselves, but them getting the dangerous stuff out and causing harm to others who actually heeded the warnings. You'd need extremely strong radiation

If you're talking about 10,000 years in the future it may not matter so much since the radioactivity would be much less after so many half lives.

Exactly. You can't rely on the radiation itself as a deterrent.

How about actual corpses arranged so that they look as if they died violently?

Not the exact message, but definitely should give pause to anybody visiting the place in the far future.

"Somebody made that place super dangerous to get into. There must be something really valuable in there!"

I'd go the other way... Give the impression there was something really valuable in there.

Have a main chamber with a solid pedestal in the center. Then, on walls throughout the complex, depict some golden treasure that never existed resting on said pedestal using wall art.

Imagine a scavenger, with a clear vision in their head of what the chamber looks like, arriving in an empty room: "Someone must have already raided this place."

Meanwhile, the nuclear material is under that chamber.

Then don't make the place inaccessible. If you're warning people about a threat, you need to show them where the threat is, not hide it.

I suppose it's because without any context a pictogram like this could be interpreted as a letter or a symbol, not an actual warning. It's not immediately obvious looking at this that it's a human hand being stabbed. Also it doesn't convey that the thing stabbing you is invisible. And if you figure out that the stabbing thing is burried then you might want to dig it out to use as a weapon against others.