I think that, the mainstream belief, is that the current president (whomever it may be) is viewed as the top dog who's word is law. This is rarely the case. Does he have large influence on decisions? Of course. Is he responsible for every decision, good or bad? No.
The president does not make decisions or laws on his own. The bill or law must first pass through waves of congress, senate, and other representatives before passed along to the president. The president then has the power to veto that decision, or pass it. If it is vetoed, it is passed back to the congress who may veto the president's veto, and enact the law anyways. So, it is more or less balanced out. The president's power is, in fact, limited.
Why is only the president blamed? Because it is easier to gang up on one man than a group of men. Everyone knows the current President, but how many people do you think can name three congressmen? People see a mistake, people blame the closest most obvious possible cause, usually the one that doesnt require thinking or research of some sort. Hence, the president receives blame.
The President is constantly being blamed on Social Media and the News channels and this is just adding fuel to the fire in a nation that is already extremely divided. Celebrities are also contributing to this with Social Media, Award Ceremony comments and on the news also. Also fact checking is great but why does it's website put left favored things all over its homepage? News such as Yahoo, MSNBC, As well as several others put much more Liberal opinions than Conservative views/opinions. Media doesn't seem to be held accountable for their reports being misleading. The Journalistic News anchors insert their opinions on News instead of just reporting it.
Obama is currently in gridlock. He does not want to go to war because of the past mistakes made in the previous administration. Their mistakes also outweighed his to the point that the blame was shifted over by partisan Republican party members and supporters. An example being the so called Benghazi 'scandal' where a few American were left behind and killed in Libya. However many more incidents like this occured under the Bush administration and are ignored. So how is Obama the source of all woes in the United States when his own Congress refuses to support him at all.
The president's powers are limited, and many other people are in charge of policy making. Sure the president basically makes an oath to take the heat when stuff goes wrong, but it's not right. What the president can do is very limited, and the economy is the result of many presidents.
They can and should be blamed for the policies they suggest and the way they try to sway the american people.
Obama for example has been surrounded by a congress determined to see the government fail and lay the blame at his feet. The only responsibility he has in this is that he was unable to persuade the people to force them to stop (not that those willing to completely shut down the government are likely to listen in the first place). Almost all of the blame here falls on those who failed to negotiate, or who negotiated and then went back to their original position while the opponent has to start at the previous negotiated point.
On the other hand the blame for the Iraq war falls for the most part on the shoulders of Bush as he alternatively blamed them for 9/11, claimed they had weapons of mass destruction that they could unleash against the US, and that we are saving the people; all to justify the war while calling detractors unpatriotic and against the troops.