A baker can not change the prices over day.
What he has put into he window in the morning is in most countries of the world binding till the evening.
So my example makes perfect sense and your does not.
Now lets talk about a gas station, which can change prices arbitrarily (more or less) ... then you had a point.
Nevertheless: that is not the market in both examples we have a single bakery and a single gas station.
Uber on the other hand tries to change prices for the whole market it has access to: that is unfair towards the customers.
If there is much more demand than there is supply, raising prices is a very legitimate (and used) method of ensuring efficiency, which otherwise would be much lower. Higher prices don't increase efficiency. They only rip either the customer off or make the supplier rich or both (or somewhere a middle man).
Lower prices indicate more efficiency ... that is a no brainer.