This is the wrong comparison though. Merchants offer bulk discount to incentivize customers to buy more units of whatever they’re selling. I can’t buy more than one seat on the plane of its just me traveling. I can’t utilize the ‘bulk discount’ if I’m not traveling with anyone
Group discounts are pretty common for all kinds of services, they are pretty comparable to bulk discounts for goods. You might not use two seats on your own but you might bring along a friend.
But they could absolutely move to a system where the vast majority of filers could attest that the numbers IRS already has are all the income they have and be done with their returns. Then only the subset that has other income or adjustments would need to do a more involved filing.