Not employing you at a whim does more than just make you work at a different place the next day.
You now need to spend an undefined amount of time looking for a new job.
You need to find that new job soon, because your old job was tied to important things like health insurance, without which any accident could bankrupt you.
It was also tied to less important benefits like vacation time and seniority, which reset at youe new job. Previously had 3 weeks off available after working for years at the same shop? Back to 6 days. At some places it may also be tied to a car, computer, or phone plan which you have come to depend on.
Not to mention that getting fired is a black mark on your ability to get a new job in the future.
Employers like to structure things such that they can replace an employee with little impact. They have people whose job it is to manage that process. To an employee, even an unusually well prepared one with savings and a plan and a current resume, losing one's job is a catastrophic event that can only be easily withstood a handful of times in a career.