https://www.homedepot.com/p/Milwaukee-100-ft-Bold-Line-Chalk...
I love youtube manufacturing videos, saw one where they made a wooden Japanese one that has like a hopper of soot where the string is pulled through.
Consider the planets. They noticed that among thousands of stars these five move funny. Of course it helped that most of them are very bright and don't twinkle. On a clear sky even Sirius often doesn't twinkle though.
[0] https://www.tajimatool.com/product_category/mt/#chalk-rite
A laser beam* across the room will show the defect in the string straightness. It's more than good enough to fool human eyes, which are not good at judging slow gradients (such as all the touristy "mystical anti-gravity locations" where balls roll apparently uphill). Therefore, the snap-line is good enough. But not perfect.
* Gravity of course still affects the laser beam's straightness, but on a level good enough to fool electron microscopes, so we can give that a pass.
If the purpose is also to measure the distance between the 'pegs' and one uses the length of the cable in between, then it can be a problem. That's why survey chains are expensive.
If we get real picky, no physical method will really be accurate because straight line is a mathematical abstraction. It can only be approximated in the physical world, much like a circle.
Light paths come closest, although they 'bend', they bend in a way that is 'straight' with respect to space-time.