Because there are only 24 hours in a day? Because we have also 100 other things to do? Like a denoiser, to take an example at random… Or ftupid Maya 2016.5 stuff… Will get to it, but in the meantime, this is exactly why you have OSL.
Keep in mind that implementing glossy Fresnel properly is not possible with OSL, it must be done in the BRDF implementation itself. You can only get an approximation with OSL.
Best regards,
Vlado