Here’s a rather neat video showing how you can synchronize 5 metronomes that were initially beating at different frequencies (I said how, not why). From my somewhat primitive understanding of high-school Physics, I’d suppose it’s something to do with natural harmonies and maybe resonance? Something about each (originally isolated) individual systems are brought to form one whole system that can only have one frequency.
Or something like that. Oh well! Still, a rather interesting video!