The Black Death is said to have made labor more expensive and the peasants free. But why hasn't the opposite happened?
In economics, there are two major effects; the income effect, and the substitution effect.
All other things being equal, if there are fewer laborers, there is a lower production of food or other goods. That's just the income effect.
On the other hand, if you keep the amount of land constant and reduce the number of people, the amount of land per laborer rises. The result is that laborers gain and landowners lose, because there is more land chasing fewer laborers. That's called the substitution effect.
Before the Black Death, western Europe was arguably on the brink of a Malthusian trap. Postulated by by the Reverend Thomas Malthus, this was a theory that believed that mankind would reproduce to the point of pushing itself to the limits that the earth could support, which is to say down to the bare subsistence level. Indeed, by about 1300, western Europe was at a point where people were working land of very marginal quality (relative to the technology of the time).
The Black Death changed all that, by relieving the pressure of population on the available food supply. The latter went down of course, but the population went down much more. Compared to "earlier," there was more food per peasant. That gave peasants more choices, pf which landlord they wanted to work for. Specifically, the landlords that either paid better or demanded less service.
You appear to wonder, why weren't peasants more likely to rebel before the Black Death, when there were more of them. That's not how society usually works, and that's why Communism, which preaches "dictatorship of the proletariat," usually fails. A large number of peasants seldom bands together to demand that landlords give them a better deal. Instead, when they can barely survive, they compete with each other and "suck up" to the "powers that be" for a chance to survive.
The Black Death empowered the survivors. They were survivors, after all, who had won the competition with their fellow peasants. And fewer of them meant less competition in their dealings with landlords.
A recent, modern example of the foregoing is the fact that most European and some Asian countries enjoyed economic booms in the 1950s, after World War II. The war was a great tragedy for those that died. But the survivors (and their children), made out like bandits, because there were fewer people to share in the technological boom and freed up industrial capacity.
In addition to dwolfeu's answer about supply and demand, it should be noted than in agriculture there usually are two limiting factors: land and workforce. Due to supply and demand, when land is scarce and workforce is relatively abundant, land is expensive and wages are low (Europe in 19th century), while where land is abundant and workforce is scarce, land is cheap and wages are high (North America in the 19th century).
The population toll of the black death reduced the workforce while keeping land supply constant, and therefore wages rose - at least they rose when measured on agricultural products.