Because the physics is fairly well know, as I understand it. See for example these[1][2] articles from Microsemi and TI respectively, or this review paper[3] for power electronics.

[1]: https://www.microsemi.com/document-portal/doc_view/124041-ca...

[2]: https://www.ti.com/lit/an/sprabx4b/sprabx4b.pdf

[3]: https://www.mdpi.com/1996-1073/17/18/4589