Why do some manufacturers reverse preamp polarity in the first place?

Agnew!?! Ha!

Oh, this is about polarity? Okay, that is appropriate.

By the way, I can't hear a difference when I switch my system's polarity.

The reality is that there are no standards for preamplifiers like output voltage, polarity, etc.
Preamplifiers are the biggest offenders on all things.  Most are flawed to very flawed.

Here is all you need to know about polarity. 
http://www.6moons.com/industryfeatures/polarity/polarity.html

Cheers. 

I get an uncomfortable feeling when the 6 Moons author states, “there’s a 50-50 chance a given recording features Inverted Polarity.” I’m mean how does he know?” I would sincerely like to see someone, anyone, compile a list of say fifty audiophile recordings, LP and CD, with Polarity for each. That doesn’t seem too difficult. So far all we have is George Louis’ Polarity List. Which is great that he compiled that long list, but I have trouble buying into it, primarily because he concludes the percentage of CDs that are in Reverse Polarity is so high. But 50-50 assumes no audio engineers or mastering engineers check for Polarity and I don’t buy it. I’m not saying it’s impossible, just the probability is low.