Because the basic income money isn't given to purchase any particular good. People can spend it on what they like. If the person gets to choose between spending their $5000 on a laptop or plane tickets, supply-and-demand says laptops will stay at their marginal cost.
On the other hand, it's supposed to be an amount of money that is enough to subsist on. If there's a relatively predictable perception of "subsist" in terms of proportional allocation to categories of goods in a basket, prices for goods in this basket might start to track their respective allocations.
On another note, who says the rational thing to do with a windfall is to go blow it on something just because you can?
This is basic supply and demand - If I give everybody in the country $5000 to go buy a computer, over time, computers will start to cost around $5000. If I give everybody $400/month to spend on food, over time, the cost of groceries will rise up to something more than $400.