Because this game is so fun.
For example, i play WSR swordfight and stuck at lv 19 now.
It wasn't hard to pass through lv 1-10, difficulty has been rise a little bit at lv 11-14.
But when i reach lv 15 everything became much harder, it took about 3 days to get me to lv 18.
And it took 5 days to pass lv 18, now at lv 19 i don't know how long it takes to pass but we'll see.
(I playing by swing Wii remote like a real sword not waggle my wrist, and play about 1-2 hrs a day before i exhausted.)
other sports also really well done too, i believed i can play this game forever.(or at least until they release WSR2.) XD