Why not calibrate all display devices to sRGB?
Simple answer: because it costs. :) Some higher-end monitor models are indeed calibrated at the factory (and come with a calibration report sheet) but it would not be cost-effective to do so for all models in the product line.

Color reproduction varies even between batches of the exact same panel model, so it isn't possible to just use a set calibration for a given monitor model, it need to be done on a unit-by-unit basis. So to save costs, they instead determine the color reproduction curves of the "average" panel and use that for an ok but inconsistent result.