An interesting question. Let's take theta waves. If I recall the definition properly, that's 4-7 Hz. Your inner ear does not respond to anything less than about 20 Hz. The idea behind binuaral beats is to introduce for example a 100 Hz signal R and 104 Hz signal L. You can hear these signals even with one ear. BUT, your brain responds to the difference in frequency. Essentially, if you do the math, when you combine these sinusoids you get a signal with frequency components between 100 and 104 Hz but with a 4 Hz amplitude modulation. Your brain entrains to this low frequency amplitude modulation. It's that simple.