If you want a 1 GW solar farm you need to pay people per hour which is essentially per panel to physically install them and pay for mounting equipment etc. At 20% efficiency you need 5% more of that labor and mounting equipment than at 21%.
Not just more panels but more wiring to take care of. And more square meters of panel to clean.
And more ground to buy for a given yield.
Ahh I’m with you, i mistook your comment to mean that they were positively correlated with installation prices as opposed to negatively correlated.
At least in my house in Mexico, solar panels were mounted on custom made structures that maximize your house's power generation. That can't be cheap in a high labor cost country. The panel might be cheap but mounting it is expensive.