Because they can!
Some people want everthing to "match" so why not make more profit from the stands than you do the speakers? Sure, most people won't purchase the stands but for those that "matching" is critical--they willl pay the price. Go to Best Buy and buy a TV, they always push for a $100 HDMI cord to "make it better" because they make more money selling you a cord than they do from selling the TV. Same with cars, they don't make much selling a basic car but clean up selling the "upgrades" and toys, as you know. Why should speakers be any different? If people will blow $100 on a $5 HDMI card, why not make that cash on speaker stands, wire or whatever slse?
I have a speaker stand--it is a 4 legged bar stool made out of oak and looks really nice. Not only will it hold my speaker, I can use it to hold up big, fat butts to house the inlaws when they visit. Those creatures come to visit, I put a sand bag on the bottom legs supports so their hell spawn won't knock over the bar stools and gravity check my speakers. When I reconfigure the HT system, I could eliminate the need for those stands and re-use them for stools--for the win.
The last time I went into an audio only store (decades ago) I wanted a pair of $2,000 speakers. The salesmen was more interested in if my amp "was good enough" for hhe speakers and that I really needed to "upgrade my cables". After all, you need to spend at least 10% of the total system cost on cables so it "matches".
I tired of the audiophile preacher, did not want to join the audiophile temple of wire so left the store without purchasing the speakers. I don't purchase floor mats or undercoating/paint protection either but I don't blame them to try. We all have to eat and pay billls and accessories is where the money is. The audiophile outlet became a pizza joint within 2 years and they were pushing bread sticks--same game.
No worries these days, I just click the icon and my goodies get shipped to my door. No more pushing cables, power conditioners, speaker stands or the "TK421 mod" to improve the bass (name the movie!) Since the stores have to compete, they tend to cut their margins on actual equipment and make it up with accessories like speakers stands, wire, gimmics, DACs and other useless but very high profit items. Along the lines of floor mats, french fries and wire--sell the highest profit items first is rule one.
$500 for a speaker stand? I would love to hear the salesman try to sell that...put your hip wadders on.
Now the TK421 mod to double the bass? Now THAT is worth the upgrade!