Why is the "serpent" in the Fall believed to be literal by many? We know there is only suppositions that Satan was probably the "snake", but nothing solid proves this.  However many times, even today, we say "that bitch slapped me",  or that "dirty rat" hit me on the head... and so forth, neither of which assume we are speaking of a dog or a rat respectively. 

There is no reason ignore this of the "snake" in Eden, as no scripture says otherwise. Keep in mind there are many things "cultural that are left out when books of the Bible were written. We may say "I drove to the grocery store", yet what is the word "drove?" How fast? etc.. But we leave that out in the paper, because it is obvious. This may be true of the Serpent.

Further more you can see there is a hint in Gen 3:14, that whoever/whatever Eve was conversing with, was already UPRIGHT - standing, or flying if we are dealing with Nephilim. God cursed "it" to the ground on "its" belly. To eat dirt for the rest of "its" life.

so it is quite possible that the dirty rat conned Eve. I have not found any scripture that says This is Satan, Are the churches teaching it literal? If so why?
The Bible employs animals as descriptions of character, as we do also.

In the verse that you allude to, it expressly says "the" Serpent and draws a distinction from the beasts of the field.

Now the serpent was more subtil than any beast of the field which the LORD God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden? 

Elsewhere we are informed : 2Co 11:3  But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ. 

(You notice, through his subtilty? more subtle than any beast of the field, the physical animals)

Rev 12:9  And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. 

(You notice, that old Serpent?)

Rev 20:2  And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, 

2Co 11:14  And no marvel; for Satan himself is transformed into an angel of light. 

So we can see that The Devil, Satan appeared to Eve as an angel of light.
Consider too, apart from the fact that animals do not talk, would Eve really listen to or take any notice of an animal subject to her, over which she had dominion?

When the Serpent is cursed, he is cursed below the creatures of the field, that is relegated to a lower position, and limited to the belly, the seat of carnal affections,(Php 3:19  Whose end is destruction, whose God is their belly, and whose glory is in their shame, who mind earthly things.) and restricted to the despicable and execrable state of perdition in feeding off man, created from dust. (Psa 14:4  Have all the workers of iniquity no knowledge? who eat up my people as they eat bread, and call not upon the LORD. )

And the Serpent was set to bruise Christ's heel, which he did through the crucifixion which also bruised his head. Christ's crucifixion accomplished salvation and victory over the Serpent, as it were, mortally wounding the Serpent.