I rarely pay attention to those companies but I did notice that Pepsi has another new logo, how many times have they changed their logo in the past 30 years? 5? 10? 15?
As large as those companies are, I think that it is just a matter of keeping their name out there to keep reminding you that they are still there.
Smaller companies need to have the better marketing to get you to try their products so maybe you will become a loyal customer.
I actually drink Diet Coke, not really for the taste, just because I hate drinking water all the time, need something different sometimes.