In regards to PS5, there's a rumor that placing it vertically might eventually cause the liquid metal slip down due to gravity, leaving the upper part of the CPU uncovered, thus leading to overheating and other issues.
I know most would think "Sony space engineers should know better than that when designing the console", but honestly I wouldn't take the chance. When my PS4 was positioned vertically with the official stand, it made a jet engine noise within minutes of being on because the lateral intake was being obstructed in that position (even with the stand), and that was also designed by Sony geniuses. The minute I put it horizontally the console got super quiet again.
The PS5 also expels air where the base is attached in vertical position; placing it horizontally would leave that part completely uncovered, with more room to breathe. I would also argue it would prevent much more dust getting inside the console, as the massive vents won't be right on the surface.
Besides, vertical consoles are too freudian.