Let's say you put yourself out there. Kiss the thousands of babies. Listen to everyone's problems. Put forth your agenda. Take all the heat and lose your privacy. Beg for donations. Glad hand the party elders. Smile so much that your mouth hurts. Etc.
Then after all that you win! Woo-Hoo! You are now an elected official prepare to be obliterated by the same people that supported you.
The principle you see espoused here is not only "What have you done for me lately.", but also "Don't come crying to me when things get tough."
No wonder so many are retiring or turning away from the progressives. They aren't taking you for granted as supporters. They have taken you for granted as the disloyal opposition!