Chiplet yield
The main economic point of chiplets: by building a processor from several small dies instead of one large one, yield improves dramatically, since a defect only ruins a small piece instead of the whole expensive chip.
Mechanic: defects are random per wafer, so the probability that a die contains a defect grows quickly with its size. An enormous monolithic chip therefore has low yield (and every lost chip is expensive). Splitting the design into small chiplets makes each piece more likely to be defect-free, defective pieces are thrown away cheaply, and you can also mix nodes (expensive cores on an advanced node, cheap I/O on an older one). This is the core of AMD's Ryzen/EPYC strategy and a big reason for their cost advantage over large monolithic Intel dies. Trade-off: it requires a good chiplet interconnect and advanced packaging. Related to wafer yield and chiplet interconnect.