Because the important thing is to have the main tile, I find it curious how this is always the case. The trick is that you can choose which of the two rejoining paths will have the incomplete chicane. Choose the wrong one and it won't work
It is a little more complicated than that. There are two tiles that fundamentally matter: the entry tile, and its corresponding main tile. With them both, the game can figure out the exit tile and the exit direction, which in turn give off the entry tile of the next element. In the symmetrical split, the entry tile is shared by both chicanes, so only one of them can actually be part of the track, with the fake path being needed for the other one. In the symmetrical rejoin, that is not a problem because the entry tiles are separate.
For instance, this is a rejoin just like the one from Z217, with a complete left/east chicane:

Contrast with this other rejoin, with two incomplete chicanes:

The second rejoin works just as well as the first one, even though the exit tile of its left/east chicane is actually a filler for the right/west one.
One potential obstacle when doing this kind of trickery is that the main tile is always the northwestern one, regardless of the element orientation. That means a rotated or mirrored symmetrical split/rejoin won't necessarily work without further adjustments.