It is because of just how far off the Murderverse’s interpretation of the characters is. We want Warner to do better, but now they are so deeply entrenched in the Murderverse that I can see no way for them to unsuck their cinematic universe.
I would love to see an awesome Batman and Superman on the big screen, but for…

Or he could have just kept the spear on him. Instead of sticking it in the ground in some random ass warehouse

Or how about this one... why did the world’s greatest detective not take TWO SECONDS TO INVESTIGATE SUPERMAN TO SEE IF HE’S REALLY EVIL OR NOT WTF.

Couldn’t he have just made a kryptonite bayonette for a rifle that fired kryptonite bullets?

We’re going to have a great deal of trouble trying to impose rationality on this goofy move, but there are two choice:

Devil’s advocate - it’s not like Nolan’s Batman was much of a detective either.

Generally speaking, Marvel movies are so fun that you forgive most plotholes and missteps unless they’re particularly egregious. They’re not immune from scrutiny, either, as proven by the many critical pieces on Age of Ultron. And even Age of Ultron was more fun than BvS: Dawn of Justice.

* edit * nevermind - I see someone else found my image below.

Better yet, why didn’t the World’s Greatest Detective figure out what both Lois and Luthor figured out: that Superman is Clark Kent?

Because Batman doesn’t shoot guns! Except all the guns he shot in the film...