Here's why:
* Tectonic plates aren't rigid: They are massive, but they can bend, flex, and break. This means the boundary isn't a single, sharp line.
* The boundary is a region of activity: At plate boundaries, you'll find earthquakes, volcanoes, mountain ranges, and other geological features that develop over a relatively broad area.
So, think of a plate boundary as a zone of interaction between two or more tectonic plates, rather than a single, fixed point.