The switching is not required on every light and the code has not been adopted everywhere. If manufacturer "A" is paying someone to wire a switch, while manufacturer "B" is not paying some one, manfacturer B will likely sell their lights for less so who do you think is going to sell more lights? This is part of the ugly side of business. If you have been in the trade long enough you will see the qualitity in construction of fixtures and everything else come down in qualtiy. The cheaper it can be produced, the more proifitable and competitive the company is. Maybe in time, you will see it pre-wired at the factory.
Another posibility is factories will need to retool and this disconnect is a interim fix. Retooling even for the smallest change is way more complicated then many realize.
