Why do so many Raiders behave like psychopaths?

While Virusbomb's answer is correct that the underlying reason is due to gameplay mechanics, I think there's also an in-game explanation given--albeit, not one as explicitly spelled-out as some of Fallout's other worldbuilding pieces.

Psycho is a chem in the Fallout-verse that's very commonly found in Raider camps. It's often associated with Raiders even more than other chems--Cait, a Fallout 4 ex-Raider companion, uses Psycho as her chem of choice.

She's become so addicted to it that normal methods of recovering from addiction are no longer possible for her.

In Fallout 3's Operation Anchorage DLC, you see how this drug was initially designed: its initial purpose was to increase soldiers' effectiveness and aggression in battle, and it is described as having long-term side effects once taken. Doctor Adami, the physician administering the drug, says in her holotape:

I've read the early clinical trials, and there are certainly some possible side effects -- dementia, psychotic aggression. And of course addiction.

Additionally, while Tactics is no longer considered canon, the fact that Fallout Tactics contains the perk Psychotic with the flavor text:

Your body has mutated to adapt to psycho stimulants. Effects of Psycho are doubled and the effects of withdrawal are halved.

Additionally suggests that Psycho is, in-universe, meant to cause long-term irreversible aggression and psychosis if overused. The game's suggested intent seems to be that Raiders, with their culture of using and abusing chems in general and Psycho in particular, tend towards psychosis and violent aggression more than the rest of the wasteland's population, and that this damage doesn't reverse itself even for Raiders who aren't actively using.

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".