As to the "why", it's probably because some influential program or OS's UI started using toggle switches and then a lot of UI designers felt a need to follow al

As to the "why", it's probably because some influential program or OS's UI started using toggle switches and then a lot of UI designers felt a need to follow along. It's unfortunate that bad UI designs are so contagious. (Some of my own pet peeves are skinny scrollbars on desktop apps and the fixation on making whitespace-infested UI that has no borders between different sections of the UI.) Going back to toggle switches, the look of the switch when it's on is symmetrical and nearly identical to when it's off, barring a rotational transformation and sometimes a subtle color difference. Checkboxes on the other hand look very different when on or off. I think one guiding principle for UI should be that different states of the UI should look significantly different from each other. In the case of toggle switches, the difference is just not significant enough.