One reason is because mirrors (freespace optics in general), are limited in how narrowly confined the beam can be. Generally speaking, waveguides can have cros

One reason is because mirrors (freespace optics in general), are limited in how narrowly confined the beam can be. Generally speaking, waveguides can have cross sectional area of ~ λ^2 or less, while freespace optics can only get below that for some small focus volume. So a chip with mirrors on it would have to be much much bigger to handle the beam width of all the laser light.