Many entrepreneurs believe they are quite good at hiring because that is what it feels like when the company is small. Everyone sits close to each other, hears the same conversations and shares roughly the same understanding of what needs to be done. If someone starts working in the wrong direction it becomes obvious immediately and can be corrected in passing. In that environment almost everything works automatically and it is easy to believe it happens because you chose the right people. I have heard many entrepreneurs with teams of four or five people talk about how amazing their team is. How lucky they have been and how well everyone gets along.
What they do not realize is that it is the situation that works, not the hiring. When the company grows the conditions change quickly. People no longer hear the same conversations, work becomes divided and individuals start working more independently. That is when you suddenly notice that some people push the work forward while others spend a lot of time on things that do not actually make much difference.
One of the reasons you are bad at hiring is that you hire in the wrong way. You meet someone, talk for a while and try to form an impression. If the conversation feels good and the person seems competent the decision feels fairly obvious. You also often assume that others will work roughly the way you do, that they will see what matters and push things forward in the same way.
That is not how it works, but you usually will not notice until the company has grown a bit. Only then do you realize that you made some truly bad hires. Bad hires happen all the time even in organizations with processes, tests and HR departments. If you are making decisions after a pleasant conversation the risk is quite high that you are overestimating your own ability to judge people. Many entrepreneurs only reach that insight once the company has grown large enough for the weaknesses to become visible.