Why do so many Raiders behave like psychopaths?

There is no in-game explanation because it's how the game has to be. The mindless enemies you kill have to seem like badguys to make it easier to kill them nonstop. And it's much simpler for a developer to make a one dimensional army of crazy evil people for you to kill instead of giving them all logic and rationality.

A series like Fallout has many factions with lots of complications and gray areas, but there needs to be some factions as fodder that will always be enemies to give the game a presence and variety of challenges.

In the age of complete paranoia and anarchy most good people of the Post-Apocalyptia travel extremely rarily if at all, and those that do are usually nothing but bad business.

This makes me think that most raiders attack on sight only because they expect to be attacked head-on otherwise. Note that most raiders have camps off roads exactly because they want to be left alone and undisturbed in between their caravan robberies.

As for people's remains, they may very well serve as a warning sign to stop casing that large shiny chest in the window, turn around and leave immediately. (If you came across a head on a pole in the real life, what would be the first thing you would do?)

They may also be "handy" as mutiny deterrents that would instill fear and obedience into their own members and act as a not-so-nice way to say "this is where you end up if you disagree with our leader".

While I'm not aware of official comment on this, I believe this is a combination of the original authors' view of humanity and a little bit of social commentary.

With a lack of central authority, anarchy from societal collapse can lead to tribalism. If your view of humanity is that humans are primarily self-interested and that society is what provides a moral compass, the end result could be a bunch of violent, warring tribes claiming whatever they can take by force.

As far as the "heads on a pike" aspect of raider camps, using gore as a warning sign has been used throughout history by civilizations big and small. It is traditionally a form of capital punishment used as a warning to others not to follow in a criminals footsteps, but in Fallout I'm assuming the raider tribes would be using displayed gore as a combination of war trophies, warning signs to outsiders, and a display of power.

I was not able to find a lot of citation for this stuff (the current political climate in the US is throwing searches for tribalism way off), but here was what some quick Wikipedia searches picked up:

• https://en.wikipedia.org/wiki/Neotribalism
• https://en.wikipedia.org/wiki/Gibbeting
• https://en.wikipedia.org/wiki/Decapitation

The Fallout series are far from realistic, so if we want to look at psychological reasons why raiders, supermutants or ferals behave as they do, I think the most likely, although boring reason, is as simple as "here's some evil things you get to kill" with no deeper meaning. It's part of the Fallout setting to go from deeply immersive and psychological one moment, to shallow shoot-'em-up with excessive violence the other moment.

Ignoring that, there are some possible explanations:

Drugs

Might explain it at some extent, as mentioned in another answer. Raiders across the wasteland are definitely fond of drugs. However, the psycho drug is just one of many.

Intimidation

I'm not so sure if grotesque decorations are necessarily there for scaring off unwanted visitors. Sure, it might scare off burglars and scavengers, but it might as well attract dangerous wildlife: various insects, yao guai bears, supermutants, deathclaws. To decorate your shed with unburied corpses rather seems like a big risk.

Radiation

As always in Fallout, radiation is to blame for a lot of bad things. We have the Children of Atom, who are religious fanatics and often quite insane, which is at least partially explained by their repeated exposure to severe radiation. They are occasionally using grotesque decorations like skulls and corpses. And in Fallout 4 - Far Harbor, there are raider-like groups called Trappers who have been driven insane by radioactive fog.

So the in-game lore repeatedly tells you that exposure to too much radiation might make you lose your mind. Or grow an extra head for that matter. Not very realistic, but makes a whole lot of sense from a Fallout lore perspective. Radiation is everywhere.

Raider culture

The Fallout 4 Nuka World DLC provides a lot of insights in raider mentality. Here you end up close to three very different raider groups: The Disciples, The Operators and The Pack. They are more powerful compared to the average raider groups you typically come across. In a way, these three groups represent different aspect of raider mentality.

The Disciples fit the typical psychopath cliche: fond of torture, knives, blood and gore, with the typical grotesque raider decorations. The game doesn't mention why they are a bunch of psychopaths, just that they are. And because of it, they are feared and well-known.

Raiders definitely have a kind of sub-culture. To be a good raider, you should kill without hesitation, be dangerous and strong, approve of violence and fighting, be greedy, be fond of drugs and booze etc. Embracing these twisted ideals gives status and fame among raiders. Which in turn could help raider groups when recruiting or bullying settlements into paying them.

So grotesque decorations might just be a way to show that you are dangerous and live up to the raider ideal.