You haven't seen them because you obviously don't watch any of Trump's rallies.
Inconvenient fact: Trump calls Obama "disgraceful" and "a disgrace" and "the worst President" and "a disaster" -- and many, many other slams -- multiple times in every campaign appearance.
Did he blame him for losing Iraq? Did he blame him for all the terror attacks that happened on his watch? Get back to me when he is as hard on Obama or Hillary or Bernie as he was on GW or other Conservatives...