Why does my baby sneeze whenever we leave the house?

Brightness or breeze. Great question! very likely this is due to how bright it is outside compared to inside. Some babies sneeze in bright light (sun, looking up recessed lights, doctor exams) in the first few months, just like if some adults "need" to sneeze, looking at the sun can bring it on. Could also be a breeze or pollen but my money's on sunlight. To test, see if it also happens at night.
To blow their nose. Babies use to sneezing to clear their little noses of tiny little particles that are in the air.Your house may be very clean, so the baby doesn't need to clean out her nose in the house much, but at various times of the year there are lots of particles such as dust and pollens that are in the air outsie. She can't say "mom, give me a tissue to blow my nose" (nor does she know how), so she sneezes!