Why do monitors shift color over time?

I’m a professional photographer and I calibrate my computer monitor regularly to make sure the color is correct, but I want to know WHY the monitors shift colors over time. I’m taking an inkjet printing class right now for my Master’s and I asked that question and no one knows why. Seems like this is a problem that would have been solved when we all switched over to LCD screens. Is it something in the technology or the environment?