why don't they get it?

I have a daughter whith a cellar and attic full of old lamps, vases, chairs, and god knows what else, which she picks up at tag sales. No audio equipment. Your urge to collect reminds me of her. Of course, I am not like that. All my things are good stuff.

Of course, back when the Kenwood tuner you bought was made, people said the same thing about equipment that was made in the 50s and 60s - 'built when pride and sound were important factors and not these fancy transistor things, but good reliable tubes'. Plus ça change....

Most of those saying that, BTW, were male, so this is not a female thing.

I was told the bathroom was filthy.

I went in, looked around, and had no clue what she was talking about.

I guess I just didn't "get it".

I can't wait for Elizabeth to chime in

Is it that you cannot understand her different perspective, or that you chose not to accept it? Not trying to be a smart-ass, but people are free to chose which preferences they like. With education (like your description of the quality of the unit) there can come understanding and sometimes appreciation. But, don't count on her being wowed by audio.
My wife has a very benign attitude toward my passion for this hobby. That is not likely to change over time. The truth is that many women think such fawning over equipment is infantile, or at best juvenile. They also often think that newer equipment must be better, and in most cases, quality to price point being equal) it is.
However, the "female mind" effect you speak of is not universal. Both my wife and I prefer older homes, built in the '60's and prior, since we feel they have far more character and build quality than most built today. Others, men and women, can't wait to get into a newly built house since newer has to be better, right? Some of those same people, men and women -couples, have seen their new homes get rotted windows within 10 years due to lack of air circulation, and have walls so thin/poorly built that the house shakes in storms.
So, you're right in the "build quality" assessment of the older equipment, but look at all the guys dumping big bucks for cheezy surround systems. It's not just the female mind. Most listeners will either believe they can get performance way beyond their investment or they just don't care about audio performance as much as you and I do.
Get your girlfriend on a topic that she's passionate about, such as maybe style/fashion - just a guess, since it's important to many women - and if you're not as gung ho on it as she, it's likely she'll think it's one of those "male mind" things...
One last thing; her revulsion toward the component may have absolutely nothing to do with audio and everything to do with relationships. Women think continuously in terms of relationships; if there's a problem often it's traceable back to a relationship question/issue. If her family were "collectors" (nice term for pack rats) she may have a revulsion to old and seemingly useless things piling up in her life. Worth discussing, and if you value your relationship its worth discussing and coming to a compromise on how much stuff will invade your lives.