The problem with cases that are oriented horizontally is they're usually required to be in a slim form factor in comparison to "normal" ATX or even mini ATX cases. This poses a problem for providing adequate cooling and power delivery. In face of GPUs (NVIDIA/ AMD) becoming more power hungry/ heat producing this just doesn't work out for most people. There were some attempts in the past[0] however they don't appear to have gained mainstream adoption. On the other hand, systems with low power components could be accomdated in this form factor but they most likely wouldn't benefit much from it (or just employ a laptop with a dock).

[0]: https://www.youtube.com/watch?v=nRBJ2fysc64