Fix-It Felix says something along the lines of:
I'll cover for you
We can only speculate that it's a mix of Felix covering for Ralph and the game not being as popular as it once was.
Plus as few people have pointed out in the comments, it could have been a day when the arcade was closed, we know it all happened in roughly the space of 24-36 hours, ...
In all likelihood, Fix-It Felix Jr. (Ralph's game) was not the most popular game at the arcade and Ralph's absence was not even noticed sad trombone
Consider the age of the game by the graphics and gameplay depicted. Similar to Donkey Kong or the original Mario Bros, this looks like an 80's game that somehow has survived to modern era.
Compare to Sugar ...