Why is a solar module assembled from multiple cells instead of one large cell area?
Some PV modules are indeed one single large area: thin-film modules and polycrystalline silicon modules can be done as one single large area, or as arrays of rectangular cells. The design in this case is driven by economics.
Monorystalline silicon modules always consist of an array of connected cells: and the reason for that is primarily about the science, rather than the economics.
That's because the cells are made up of wafer-thin slices through a single crystal. Making the crystal is a complex process: the temperature of the molten silicon needs to be homogenous, as does the concentration of the doping chemicals. And the silicon needs to be mechanically stable as it crystallises around the seed crystal.
These constraints mean that the cell size has tended stay in the range 200mm - 300mm diameter. There are industry developments to take this to 450mm. There has been lots of learning-by-doing, and huge economies of scale, that have brought PV cell prices down by about 80% oer the last few years. Changing a successful formula, and going back to lower production quantities, is a risk. But those are economic questions, not physics ones, so I'll stop there.