Kickstands and mountain bikes
If, by stands, you mean bike mounted kickstands, then the reason most mountain bikers don't use them is three fold:


Safety. The kickstand is usually a relatively cheap piece of metal, bolted on wherever it will fit. Its shape and style lend themselves to ending up in your wheel or otherwise damaging the bike, if they are not secured to the frame so they can't accidentally deploy themselves while riding a rougher trail.
Weight. If I pay a ridiculously large amount of money to buy a light weight, high performance machine, the last thing I want to do is pay extra to make it heavier. So that does play a part, but I wouldn't say it was the key factor.
No need for it. As you mentioned, most riders are perfectly comfortable dropping their bike on the ground wherever they pause for a minute or two, and dealing with cleaning and fixing any damage that may cause when they get home.


I hope that helps.