Everytime I check a game I see that sales of games in Japan fall so hard after it's first week to a very low point. This doesn't happen in Europe/USA where games (depending on quality and marketing ofcourse) don't drop that hard and decline much slower than in Japan. What are/is the explanation(s) for this?

Because renting games is illegal in Japan so they have to buy the game if they want to play it right away. Also, people tend to buy used games in Japan

Reaganomics

several factors:

1.) many jpn people buy the games they want first day

2.) there is a strong second hand market, which of course is fed by the many first day/week buyers

3.) in Japan are afaik still more game releases than anywhere else in the world, so shelve spaces are quickly filled with new games and old games that don't sell as well anymore won't be reordered by shops

Maybe they don't like a lot of games so they don't sell them anymore.

They don't know the concept of long legs... see?

Short legs... everywhere...

^
this must be it

I think the secondhand market is massive there for them, making them smarter than me at least >.>

Retail turns over new titles too fast and the used market is HUGE in Japan. Makes it near impossible for smaller tier or niche games to get any real retail presence.

Yep...Weird.