So your arguments are that FA can have bad day, they may had to deal with rude passengers and are tired of long travel. OK, but tell me one thing:

FAs on some airlines doesn't have bad days? FAs on some airlines doesn't have to deal with rude passengers? FAs on some airlines aren't tired of long travel? Because FAs on some airlines provide great service, are always smiling and willing to help.

Why one FA tells me "we'll have drink service in 20 minutes" and another "Sure, I'll be right back" when I want bottle of water?

My answer is: FAs on some airlines are well-trained to provide great customer service (what is part of their job) and are either required by management to do so or simply realize who pays them.