I think you're conflating one question with another. The "why" in question is why altering neural activity in that way results in clinical effects. It is not the "why" TMS alters neural activity.
From 2024:
> Transcranial magnetic stimulation (TMS) is a non-invasive, FDA-cleared treatment for neuropsychiatric disorders with broad potential for new applications, but the neural circuits that are engaged during TMS are still poorly understood.
[0]https://journals.plos.org/ploscompbiol/article?id=10.1371%2F...