I think the biggest reason is added cost. If you make a comparable amp to your competitor, you will price yourself out of the market if you mark it up $100 and include a better power cord. It also depends on the customer. It’s not like the amp sucks without a good power cable, it just doesn’t perform at its peak which many customers may not care about (not everyone is as anal as all of us here when it comes to music), and the customers who do care would rather shop around for the cable that sounds best to them (who wants to pay an extra $100 for a cable that will just sit around because you already had a better cable or you find one that better suits your taste?). Then there are the denialists - “No way I’m going to buy an amp from that company when they believe in pseudoscientific concepts like cables!” 😂 Manufacturers are in a tough position caught between cost, anal audiophiles and not turning off the denialists any more than they already are.