It’s not just the cloud cover and going through more atmosphere, it’s also the amount of energy per square meter hitting the ground. But a large part of it is most solar panels aren’t tracking and even if they are it’s usually in the horizontal and not in the vertical.

It’s impractical to mount solar panels at such an extreme angle, but it also won’t help very much since the sun is so close to the horizon shadows will be terrible. Imagine two rows of panels, once you set up the first one almost vertical, the second row won’t get any sun. And that’s if there is even a clear view to the horizon, in most places that’s not true. It’s also very hard to mount panels at such an extreme angle because the wind will catch it more easily. Mounting flush to the roof is usually preferred, or at a fixed angle with struts for flat roofs.

Because the panels don’t track, higher latitudes are less efficient, as the sun varies more in angle during the year. From just peaking out over the horizon in winter, to high in the sky in summer.