Something to consider is that smaller chiplets have higher yields than monolithic dies given the same defect rate, which can certainly have an effect on price.
Right, more precisely cut out the defects from a wafer and save as much precious top-tier silicon as possible. Plus, another benefit of chiplets is that not every circuit needs the same performance level. Save the 5nm stuff for "hot paths" and use increasingly older processes for less performance/power critical applications, because while they might take up more space, they won't unnecessarily take up more next-gen fab time. Phones are literally the smallest and most power-limited devices we use so it makes sense to make every chip in them 5nm. Laptops and desktops and servers are not so constrained.