Why are employers responsible for my actions?

it's a common law practice

if you are acting in the scope of your employment the employer is responsible.

now if you are on the clock and rob a bank - they aren't liable.

I guess they figure if the employer benefits from your labor then they take the risk of your screwing up

Because you are a representative of the company and the company vouched for your ability to the best of their knowledge.
They hire you, they train you, they vouch for your ability to do the job.

Because the customer engaged the services of the business you work for; they didn’t engage your services personally

Companies will have liability insurance for this, and they can write off the expense.

I hope that didn't happen though 👀