Why are leaders usually not threatened by violence?

[Rev. Rye]

Rich people can be a problem when they have undue influence over our government, but the system that encourages that doesn't have a brain to put a bullet in.

Yeah, even if they do get taken out, it's almost guaranteed that they'll be replaced with someone who's just as shitty as the people they replaced. That may be another mitigating factor. Going back to Lincoln, when he got shot, his replacement was Andrew Johnson. As divisive as Lincoln was in life, Andrew Johnson was a lot worse: his main accomplishments were:

• Integrating the Confederacy back into the Union, a process of Reconstruction which he may or may not have botched. 
  
• Buying Alaska from the Russians.
  
• Being the first sitting president to be impeached, albeit under a wildly unconstitutional law that was enforced exactly once, for him.
  

The bad thing is, well, all the flaws of Lincoln's administration with none of the redeeming qualities (it's worth noting that he opposed the 14th Amendment, which granted citizenship to the freed slaves). In those polls I mentioned earlier, but did not link to so I will do so here, he's almost invariably in the bottom 25%, and he gets closer to the bottom with each passing year. The only two exceptions, one was in 1948, and the other (done in 1962) had him just at the cusp of that bottom tier; I strongly suspect that if they actually included JFK, he'd have been pushed to that bottom quartile.