All factories have the entry on the bottom. Sometimes that is inconvenient. For example my setup has train stations at the top and bottom of factories. Now I've used a warehouse as chest connection and that is blocking the entry of the warehouse. So to enter it I have to pick up the factory, place it somewhere else and enter. Reverse that when I leave.

Another nice thing about rotating factories would be that you could use the same design inside with factories placed next to water. This would avoid the need to pipe water around to the right side of a factory.

The ability to rotate factory buildings would either require moving the entities inside around (which is not mod-compatible), or require having the inside and outside of the factory building in different orientations (which would be unintuitive and confusing), or only rotate the entrance (which would not be worth the effort involved). That's why rotating factory buildings is not going to happen.

If you want to rotate the interior of a factory building, you can blueprint the inside and replace the blueprint rotated.

If you have trouble fitting into the entrance, try Squeak Through.