Even on a cloudy day, when you step indoors, the level of the light drops by more than 5 f-stops. So, on that same cloudy day when the exposure might be ISO 100 at 1/60 at f/8, indoors the exposure suddenly becomes ISO 400 at 1/15 at f/4. On a sunny day, the difference is even greater.
Why is there so much less light inside? Outdoors, the sun creates a huge amount of light, and even when the sun isn’t directly hitting the subject (as on an overcast day) the sun lights the clouds making the entire sky one giant, soft light. When inside, the roof and the walls block virtually all of the light. Even if you have four 3-foot by 5-foot windows, the light coming into the room is still limited. Exceptions obviously exist, such as skylights and when direct sunlight is streaming through a window.
