Why do so many sellers take offense to offers on their listings?

Either name your price or put it up to auction.  I don't understand why this is an issue.

Just look at all the posts here, how many are about music, less than 10%???

There are threads that are dedicated specifically to music and it's probably safe to say that many of the participants in those threads spend 80-90% of their time in the Audiogon forum there.  Who knows, maybe some of them have even banded together outside the confines of Audiogon and spend a lot more time somewhere else discussing music as well as gear.

the primary purpose of this site is to buy and sell gear...though there is discussion of music, there are many other sites more focused on music and quality recordings...

I wouldn't get my "undies in a bunch" over an offer that I'd consider "low ball", when I've gotten them, it's usually someone contacting me through the email feature, so they are not really "committed" to making that offer.  I usually respond with a price I'd consider and leave it at that.

Speaking of using the email feature, I'd not at all consider an email contact as an "offer", I'd insist that the potential buyer make a formal offer on the site.  One time the potential buyer asked for my cell number, as he said he had some questions, I gave it to him and what he really wanted to do was negotiate, we came to an agreement on the price, but when he arrived at my home for the pickup, he said he thought about it during the drive and said he'd only offer $600 less than we'd agreed upon.  "No Thanks".  After that experience, I've insisted on the buyer making an offer on the site.

You're not dealing with sales professionals, for the most part. Most of the time they're inexperienced in dealing with 'customers' and way too emotional about what they think the value of their special DAC or turntable is. If I got insulted every time someone gave me a ridiculously ill informed offer, I would have been broke many years ago.