From what I've seen the extra SATA ports don't come without cost to the motherboard vendors especially if they are running USB 3.1 gen 2 from the chipset. A SATA controller (ASM1061) is used on MSI X570 Pro Carbon for example and it eats into chipset bandwidth. If you look at the block diagram for X570 Gaming Plus , MSI saves at least $5 on this chip (see page 21): http://download.msi.com/archive/mnu_.../E7C37v2.0.pdf
Edge block diagram on page 21 (note Intel WIFI 3168 and 2x USB 3.1 gen 2 added) http://download.msi.com/archive/mnu_.../E7C37v1.0.pdf
Pro Carbon block diagram on page 23 where Intel wifi 6 and USB 3.1 gen 2 ports (two) are sharing chipset: http://download.msi.com/archive/mnu_.../E7B93v1.0.pdf
With the price of M.2 drives they likely figured that M.2 is the main reason people are buying PCIE 4.0 boards. Currently there is only AMD's RX 5700 XT that uses PCIE 4.0 on the GPU-side and it doesn't saturate PCIE 3.0.
Other vendors don't exactly provide a block diagram but I suspect it is the same situation. The more chips you connect to the chipset the more you need to spend on R & D and troubleshooting proprietary drivers, it's not just the part cost on the BOM (Bill of Materials).
On X570 Aorus Elite there's a USB 3.1 gen 2 type C front panel connector on the right hand side and two USB 3.0 front panel headers on the bottom edge. (see page 5 http://download.gigabyte.us/FileList...ite-wifi_e.pdf
). On the specsheet it also lists:
1 x USB Type-C port with USB 3.2 Gen 2 support, available through the internal USB header --- USB 3.1 gen 2 (10Gbps)
4 x USB 3.2 Gen 1 ports available through the internal USB headers -- marketing speak for USB 3.0
4x USB 2.0/1.1 ports (4 ports available through the internal USB header)