End ISPs can. If you're an ISP supplying internet service to a business you can and should block source addresses that don't belong to that business, on that business's line.

It doesn't work in the "Internet core". If you're Verizon and you got a packet from Comcast that says it's originally from Cogent, how do you validate that? This router that happens to be checking the packet prefers to send packets to Cogent via Sprint, but that doesn't mean Cogent also prefers to send packets to you via Sprint. Each router can have a different preference, too. (Example scenario only)