You still have to decide who gets the probationary period and who doesn't. To figure that out you still have to do interviews and filter. Probationary periods don't solve anything except it makes it easier to fire people before the end is over.
It's perhaps not hard but the expectation if you take a new job is that you won't be randomly fired in a few months because you don't meet some unstated high bar.
Every new hire has a probationary period to it. If you go into a new job thinking you'll make it past 6 months automatically, then that's a big assumption. While it shouldn't be hard to just do your job and get past the probation period, it's hard to know if the company didn't just hire a bunch of people to get past a crunch without an intention of keeping them.
Implicitly. But there's a difference between "wow, you're a bad hire" and "you're not an 80%er."
"Does not meet expectations" is always a cop out, but a great catch-all if aren't an 80%er.