The answer is, they need slaves. The foreign workers are suitable for slaves because they don't know the labor law well.

As the birth rate is decreasing, there is a shortage of young slaves. It's a simple supply and demand situation. If you want to hire a worker, you have to pay more because there is not enough supply that satisfy the demand.

I have no idea what the future Japan will look like, but for workers, it will be a good market until Japanese economy corrupt.