Because the $1,200 checks are just an advance refund on your 2020 tax return (to file in 2021).
If you get and cash a stimulus check you don't deserve, you'll need to pay it back on your return next year.
Just like if you didn't get a stimulus check you deserved because of IRS mixup (like me), you'll just get it as an additional refund on your return next year.
Considering the speed these checks got sent out, there were bound to be errors. It's not ideal, but it's not a huge problem because everything will be properly sorted out on the actual tax returns.