If you're thinking of residential ISPs which own the IPs that their customers use, it's mostly pretty simple to prevent spoofing (or at least, prevent spoofing outside their address range), but once you get customers that can bring their own IPs, it can be more cumbersome and the default is to not care.

There are, of course, proposals and protocols to make things better, but not caring is easier, and strict enforcement is hard, especially as the bandwidth goes up.

There's also enough ISPs that don't do egress filtering that name and shame isn't effective, and anyway, what am I going to do if I'm connected to an ISP that doesn't filter? I have exactly one meaningful offer of connectivity, so that's the one I've accepted, regardless of its merits. Many ISPs have a similar hold on customers.