Why are psychopaths so powerful in Evil Within?

Is there any in-game explanation why the evil characters are so powerful inside STEM?

It is really frustrating that your enemy can do all this crazy stuff with you and your mind (sending monsters, teleport, dodge) and I hoped that once Seb understood where he is he could activate also some powers. Nope.

While in the first part it could be explained that the inventor actually had the experience to explain the power gap, the second part makes it clear that psychopaths are much more powerful by default.

Only Myra seems to be the exception and it is unclear how she did get the power to manipulate her environment. Is there a canonical answer?

ADDITION TO ANSWER:

I do not think that I am conflating experience with psychopathy. Yukiko Hoffmann, the psychologist in The Evil Within 2 says explicitly that psychopaths are much more dangerous, threatening the union project and her task was it to test candidates and prevent psychopaths from entering (yes, she failed).

It is also wrong that characters show a development to comparable superpowers with experience. Both Juli, Marcelo Jiminez and Esmeralda Torres (and the STEM team) have extensive experience with the system and do not show remotely the presented superpowers like

1. Teleportation: Instant movement of the own body and other bodies over a distance.
2. Invulnerability: Ruvik is completely immune against any weapon you try to use against him.
3. Create and destroy things at will: Ruvik has his mansion, Stefano his living camera with tentacles, Theodor his incredibly big wall of fire.
4. Time freeze: Stefano und Ruvik can freeze the time, leaving victims helpless.
5. An army of monsters and specific bosses which are incredibly hard to kill and cannot be defeated once they have you in their grasp (which are even able to raise from dead, like Hammerhead).

I think you've got some basic flaws with your interpretation:

Firstly, Sebastian does have limited powers in STEM, especially the second time around. Tatyana makes pretty clear she exists only for Sebastian, and the flashbacks to Beacon in The Evil Within 2 are Sebastian-specific. So he gets some considerable powers even if he can't exactly control them.

Secondly, I think you're conflating psychopaths with experienced users.

Ruvik, Myra, Theodore, and Stefano all knew they were in STEM, and they knew about it from the start. In addition, they've all had way more time and experience with the world than Sebastian had, both from spending way longer in the system and knowing more about the software that runs the simulation. Bonus points, they all have zero qualms with experimenting with STEM at the cost of the inhabitants.

Imagine how strong Sebastian would be with no inhibitions, six months of leveling up, and Sykes-level knowledge of the architecture.