There is no mystery here. It is expensive to bring someone new on and train them.
There is an arbitrage too by the company that once most people are comfortable, they chose stability and the known quantity they have over the unknown opportunity they would have with changing companies.
The real question is how can someone not understand that a company is not going to pay more if they don't have to? Seems kind of obvious.
What place has potatoes more expensive than tomatoes?
Outside of Asia, which is more rice-based than potato-based, potatoes have a hard-earned centuries-old reputation of food for poor people (cheap to farm, not much nutritional value). But even after checking the prices in Japan, tomatoes are several times more expensive.
Maybe you’re not looking at the price per kilogram, but price of package for packaged vegetables?