Because eventually they settle down to appropriate prices as to what people are willing to pay for them.
I just don't see why fluctuating prices are better than fixed ones. Especially without hard price caps. I don't think the economy is stable or consistent enough to justify it. You have guns selling for a few hundred caps, and bases available for a few hundred thousand. The cash flow isn't steady enough. The Rogues has a base within, hell, less than a day?
The only real consequence I can see of variable prices is poorer/newer players losing out and prices getting skewed. It's also a psychological thing. If you know you have to save up 12k for a scout, you have a definite goal to aim for, and are more likely to pursue it. Same if it's 200 caps for a dog. I mean, right now, it's about 1,014 caps for a 34hp dog in Junktown. Lot more than it says on the wiki.
It's also worth considering that after a certain point, money becomes no object for some players. If you can afford a 130k base, money isn't such a big issue. And the well off groups then throw it out of whack for everyone.
I just don't think there's any advantage to inflation. With fixed, people know what to aim for, and they're not at the mercy of price fluctuations.