I don't see why the buffers have to be specifically page aligned. But there certainly exists some hardware, that requires some specific (unrelated to page size)

I don't see why the buffers have to be specifically page aligned. But there certainly exists some hardware, that requires some specific (unrelated to page size) alignment of it's DMA buffers. But more importantly, such buffers have to be placed in physical memory such that they are accessible to said hardware, which often means in lowest 4GB of physical address space and both kernel (does not know what just allocated page is going to be used for) and user space (cannot influence page allocation) has no way to ensure this without copying pages. IOMMU should solve this problem, but then you have additional costs coming from reprogramming IOMMU.