Because people clean it constantly. I still find it amazing how many people I see cleaning when I'm in Tokyo. I was in a store mid-morning and a man was vacuuming. I walked by again in the afternoon and I noticed he was vacuuming again. In the west, that happens once/day after the store closes. There it was continuous. But nothing blew my mind more than when I was on a dirt walking trail and there was a man sweeping twigs and leaves off the trail.
As much as that surprised me, it also kind of didn't because there are people working everywhere on everything all the time. The number of concrete mixers driving around was amazing. Infrastructure is never finished and always seems to be well funded. I feel kind of embarrassed when I see Japanese people visiting my city, like they've come to some kind of pioneer town with our litter everywhere, far too many cars for the available roads and with our laughably inadequate public transit infrastructure.