Indoors the light is confined to the room and bounces off the walls and ceiling. Outdoors the light is free to spread around. With a drop lens, and mounted 30ft up, a decent amount of the light just scatters into thin air. But yeah a 50W HPS is bright as hell in my bedroom but useless on the street. If the streets were white instead of gray/black, we'd need a lot less light to see on the road at night. Just look at a snow-covered road at night. The road is very brightly lit, even by 50W HPS.
Heck, the 2000 lumen 20W LEDs on my street do a good job lighting the road when it's white with snow. They suck when the pavement is dry (and even worse when wet with rain) since virtually all the light is absorbed by the surface of the road.