European here. What do you mean? Employment contracts are not common or mandatory? Unheard of here. I painted my grandma's house for money and we still wrote a single page contract.
There are employment agreements, but they typically only cover protections for the company - not stealing IP, no competition clause, etc.
Since most states are at will employment, you don’t need a reason to fire someone, just like an employee doesn’t need a reason to quit. Both parties can end employment without cause at any time for any reason. There are some varying exceptions to this, but it’s mostly the case.
Guessing here but I think the implication is that courts in the US are unable/unwilling to enforce workers rights even when laid out in a contract. In a post-union US there is not enough organised support for workers unless they are very rich.
Obviously employees all (maybe almost all) have employment contracts
Since most states are at will employment, you don’t need a reason to fire someone, just like an employee doesn’t need a reason to quit. Both parties can end employment without cause at any time for any reason. There are some varying exceptions to this, but it’s mostly the case.