* Elderly people who struggle with technology.
* People fleeing coercive situations like domestic abuse where their abuser controls their finances.
* People with mental disabilities, for whom cash might be the only thing they know how to use.
* Homeless people, who might literally rely on cash to survive.
* Poorer people who prefer cash because it helps them budget and limit their spending.
* People so poor or otherwise disadvantaged that they don't even have a bank account.
That last group is much bigger than you think: by one estimate, 6% of American adults don't have a bank account and another 13% are "underbanked".[0] That's tens of millions of people in the US alone who don't have access to the basic financial services that you and I take for granted.
I have no idea how they do it. I'm not sure how it's even possible to navigate modern life without a bank account, but then I'm an educated, affluent professional who has no idea what it's like to be poor. Just because I don't need cash doesn't mean that no-one else does.
So basically, I think that asking "why should a business be forced by law to accept cash?" is like asking "why should a business be forced by law to provide wheelchair ramps and elevators?" If you don't think the answer is obvious, then you need to check your privilege.
[0] Source: https://usafacts.org/articles/who-is-the-least-likely-to-hav...