I can't figure out why my sims has visible seams on their neck and hairline. At first I thought it was just caused by custom skin textures.
While I was testing my default head replacement mesh, I had to make sure it was seamless so I created a full flat white skin, also replaced the default normals and speculars with flat ones. But the seams were still there. Not just on my custom head mesh but on the EA meshes as well.

Checked the face and scalp meshes in Blender and Milkshape to make sure the normals were in place.

Used the T-posing mod in CAS to make sure it wasn't caused by animations.

First screenshot is my replacement mesh with Kurasoberina's Primer skin. It's a well made skin and textures are almost completely seamless. Checked it in Blender as well, not supposed to look like that.
Second screenshot is regular EA adult male with the flat white skin.

Tried running the game with no mods as well, am I missing something?

The lighting in CAS will always reveal certain seams, but the severity likely varies with graphics cards and settings.
I use Imamii's Tabula Rasa as a default skin and the scalp seam is mostly invisible, but where the neck meets the scalp and face is definitely noticeable from certain angles. The shoulder seam, too. Because it varies as the sim is rotated, it's likely related to imperfect normal maps- the fact it isn't visible in Blender also supports that.

Best you can do is transfer normals on the seam and check if it's actually noticeable outside of CAS. If your custom mesh is higher poly at the seam than the original, you could also try including the original edge so the lighting isn't discontinuous where they transition (if that makes sense- I use my own custom feet meshes and it was easier to include the slice of faces that attach the foot to the leg from the original feet and attach my mesh to that rather than trying to blend new edges with the leg).
It may also be possible to make default replacement normal maps that are better than EA's, but to their credit it's very difficult to generate them for textures that are mapped the way the body and face are.

But this problem is not specific to my custom mesh. The second screenshot is EA's original adult male head. Milkshape is good at showing bad normals so I checked the EA meshes as well and they all look seamless there.
And also yeah it looks like this outside CAS as well.
I play with highest graphics settings. My card is AMD. I know it's more problematic than Nvidia cards but could it really be the issue here?
Are the seams visible in your game? I'm attaching the flat test skin and default flat normals and speculars that I use to test the meshes.

Yes, what I meant to express is that some amount of seams are present in all games. The rest were just suggestions for minimising the effect, which is about the best one can do when the original meshes are not always seamless themselves

The 'flat' textures are worsening the effect, though. Grey normals and speculars aren't necessarily neutral- shading a 3D surface flat and at 50% shine is bound to look a little weird. Colour and noise also play a big role hiding transitions- presence of detail blends better than absence of it, especially when working with DDS formatted textures.

You may have better results testing with no default skin, EA's white-black skintone, no normal override, and using a tiny black specular if you don't want shine.
I attached some comparisons of the differences in my game- the subtle ones in the all-vanilla image that I highlighted are normal to see at certain angles, the bold forehead one is caused by the flat skin and the flat normals, the final one is the vanilla white-black which diffuses from the subtle colour and noise.
Edit: Also, remembering that your flat skin also includes a grey normal, that seems to be the main culprit here. An object that isn't flat shouldn't be trying to light itself as if it is :p

Lastly, though it's likely not the culprit here, the way the game renders does vary a tiny bit between every graphics card model and graphics drivers- and even more for users who use settings forced through their card like through Nvidia Inspector and the AMD equivalent. Specular shine can appear pixelated, tiny seams through meshes can be revealed, so on. Just a good thing to know in case you ever see any of that!

@CardinalSims
Thank you sooo much for testing all those different combinations and providing screenshots as well. That was extremely helpful.
As you said, the main issue was the flat normals. I got rid of all of them and the seams disappeared.
Screenshots below have Buhudain's normals and flat 50% speculars. EA male with flat white skin and custom female with Primer skin.

Thank you so much for explaining it in detail.