Why do so many Americans show a complete lack of respect for Soldiers and Veterans?

Two reasons.

1) The majority of Americans have never served.

2) Service members feel overly entitled for their service. Just do a quick search of "stolen valor" on Youtube to see how service members transform themselves into jackassess. Stolen valor is nothing new, but the previous generations of veterans handled it better than the current generation. You'd be hard pressed to find WWII vets putting up signs before the 4th of July asking entire neighborhoods to be respectful of their "PTSD" despite no modern incident in Iraq or Afghanistan that involved days, if not weeks, of constant shelling.

Old veterans are more respected because they bore their sacrifice with quiet dignity. Many modern veterans demand recognition from an exceedingly distant civilian populace and wonder why they get so upset.

I think many people see that we have been fighting these wars for a long time and see no light at the end of the tunnel ! Our men and women are getting killed and still no victory for us ! Many feel that maybe it's time to get out and let the Middle East sort it out by themselves ! People are tired of this war and see the military as not doing enough ! People understand that we of the United States want to protect Israel and we do ! In my opinion most people respect the military it's just that after being at war for so long they just want an end to it whether we win or just come home !