Why not calibrate all display devices to sRGB? Why do we not calibrate all devices to sRGB in the factory? The assumption is that every monitor is sRGB anyway. So why not calibrate them to that? That would make people see same color as intended on a wide range of devices.
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.