That's a mischaracterisation. You don't hire someone (which is an expensive and time consuming process that's meant to add value to the business)
just to fire them. You hire someone to integrate them into the team so that the overall value that you can deliver increases. And it's not that I didn't "bother" to "interview them properly". It's that my (and all) filtering processes are imperfect and sometimes bad candidates slip through.
Given that interviews are an imperfect process both ways, it makes sense to keep a period of time where both parties can shake hands and leave if it's not working out. If the new employee doesn't like some aspect of the company that he or she didn't know about after working, they should have an option to leave. Same with the employer. I've had several experiences with candidates who've gotten in using fabricated credentials and other dishonest means and sometimes plainly in bad faith. Given the small size of my company and my limited resources to conduct interviews, these kinds of false positives are things I have to live with.
Now, I have two options. Keep this person (who's not going to leave and not going to accept feedback on things they need to improve on) thereby hampering the functioning of the team as a whole or let them go. I want the second option open and I'm upfront about it. I tell them openly that there will be a probation period after which we'll make the job permanent and during this period, I try to conduct regular one on ones and communicate with the new hires to see if anything is wrong on my end.
It's not a perfect system and I'd love to improve on it but it works for me, for my company, employees, and clients and there it is. And while it sounds "cruel" in theory, in practice, it has reduced false positives quite a bit. I've had to do this just once in the past few years (and that too in an area that I'm not very familiar with - marketing). Usually, being upfront about this gives candidates the message and people who are acting in bad faith make excuses and leave.