Question

why dont the plates constantly slide past each other? why do they get stuck?
magma (like in volcanoes) only moves and erupts from time to time
we only notice earthquakes that are medium strength
the rock making up the plates breaks jaggedly

Explanation:

The jagged - nature of plate - rock breaks causes friction, making plates get stuck instead of constantly sliding. Magma movement and earthquake noticeability are not the main reasons for plates getting stuck.

Answer:

the rock making up the plates breaks jaggedly