A few reasons:
1. No account needed, just send a link.
2. Speedier than alternatives.
3. Some kind of magic making it so that quickly drawn curves come out smooth and nice looking rather than jagged.
4. Open source!
5. A good UI, when so many other projects manage to screw it up.
The way excalidraw does it, it collects all the points from mouse-move events, then uses the algorithm to take those points and reduce the number of points. The reduction is done by a 'distance' parameter in the algorithm. So instead of `n` points, you now have `m` points. m < n. Then excalidraw fits a rough curve through those `m` points. This fitting is automatically done in roughjs. Fewer points and curve fitting gives the effect you mentioned.
(The reduction algorithm is implemented and used from this package: https://github.com/pshihn/bezier-points) p.s. thanks for the shoutout @swyx