Why do people insist that you can't prove a Negative?

I find that people often say that the person asserting the positive has the burden of proof. However, if a person asserts a Negative, why don't they have a burden of proof? Example: Tom asserts there is No Elephant in the room. Steve asserts there is an Elephant in the room. Shouldn't both Steve and Tom... show more I find that people often say that the person asserting the positive has the burden of proof. However, if a person asserts a Negative, why don't they have a burden of proof?

Example:
Tom asserts there is No Elephant in the room.
Steve asserts there is an Elephant in the room.

Shouldn't both Steve and Tom have to make a case for their belief? Why should Steve have to prove there's an elephant in the room, while Tom gets a free ride?
Update: Some seem to be hung up on the Elephant analogy. Here's another: Steve asserts his dead wife loved him. Tom asserts she did not. Since Steve is making the positive assertion of an immeasurable, should he have the burden of proof? Or should Tom also have a burden of proof? PS: Yes, Tom is a jerk, but this is... show more Some seem to be hung up on the Elephant analogy. Here's another:

Steve asserts his dead wife loved him.
Tom asserts she did not.

Since Steve is making the positive assertion of an immeasurable, should he have the burden of proof? Or should Tom also have a burden of proof?

PS: Yes, Tom is a jerk, but this is more about the dichotomy of positions than Tom's ill received assertion.
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