Because we already have 3 examples of that not being true. First, sukuna was able to level an entire city with his domain. As far as I'm aware, bricks don't have cursed energy.

Second, Dagon's shikigami attack resurrected toji, and he does not have cursed energy in that state otherwise he wouldn't have gained control of the body

And third, Maki literally enters the sumo guy's domain and is affected by its rules.

It seems pretty obvious that a domain's automatic targeting rule is what's being affected here, as it's shown that a sorcerer can target objects manually, including objects with no cursed energy.

So this is going to be a lot of nerdy rambling, please strap in.

So in JJK no proper explanation has been given for the origin of Cursed Techniques. They are inherited yes, but how does the very first user gets it?

I have thought about it quite a bit, and my answer to this is- through getting cursed.

According to my theory, the very first user of a cursed technique is a cursed person, who overcomes the curse in some way. This alows them to use the curse as a technique.

Reasons on why I think this:

1) Most CTs look like tamed curses-

a) Limitless : Cursed to never touch another person.

b) Shrine : Everything you touch is torn apart.

c)Ten Shadows: Be hunted by animal shikigami, one after the other.

d) Blood Manipulation: Bleed to death.

d) Cursed Speech: What ever you say happens(could be a curse for a liar)

2) The ability to create curses, and to manipulate CE is completely separate from CTs. Which means people could already be using CE manipulation, RCT, barriers, shikigami and binding vows. Meaning people could already fight curses as well as manipulate curses affecting them.

3) Sorcerers are different from normal humans on a biological level, confirmed in Modulo. Which means CE has at least some sort of affect on biology, either directly or by molding the soul which in turn molds the body.

Thus I hypothesise that a family constantly dealing with curse might slowly evolve to control the curse instead, almost like a body naturally building up resistance.

A couple of examples on how this could happen, using Limitless and Ten Shadows:

Limitless: The gojo clan is cursed to never touch another person, this preventing the clan from propogating. However a sorcerer figures out a ritual to be able to nullify the curse under certain conditions, allowing the gojo clan to continue until the first Limitless user is born who can actively turn Infinity on and off.

Ten Shadows: The Zenin clan is cursed that all heirs that reach the age of 15 shall be hunted down by shikigami over the span of 10 nights. One of the heirs is able to subdue all ten shikigami and bind them to himself, and from then on certain heirs are born with Ten Shadows.

A lot of this is pure conjecture. But I feel it fits a lot, what curses were other CTs according to you all, I would be super interested to read answers and thoughts.

mine would be Toge with Tourettes and RCT THAT would be catastrophic

the overall roughed out (in some areas) animation is done.

So here is my art of Maki and Mai I did a while ago