> Why is any company having more USB-2 slots than USB-3 these days?

Almost certainly has to do with cost and PCI-E bandwidth. Mobile chips like this only have so much bandwidth, and USB3 requires more dedicated bandwidth than USB2, not to mention more complicated circuitry that drives up price. And when your expected use case is attaching keyboards and mice, which generally are USB2 only anyways, it reduces the cost and bandwidth needed to support that many ports.