Time, money, etc., surely all play a role, but I'm not convinced that these answers are sufficient either. Probably the real answer is that it varies for everyone.
For me, the biggest hurdle was just realizing it was even possible. I had been implicitly trained for years that what you do is go to school and then get a job. The question of starting companies never even entered my mind. Thankfully, I came across some good resources (e.g., Philip Greenspun) that started to change my thinking.
Now, I do work full-time at a company, but also do my own projects in my free time. I wish I had more time, but frankly, there's a lot you can get done part-time if you just go at it. My current biggest hurdle is neither time nor money nor realization of possibility... but because I do have limited time, I tend to overanalyze project ideas before starting them...
For me, the biggest hurdle was just realizing it was even possible. I had been implicitly trained for years that what you do is go to school and then get a job. The question of starting companies never even entered my mind. Thankfully, I came across some good resources (e.g., Philip Greenspun) that started to change my thinking.
Now, I do work full-time at a company, but also do my own projects in my free time. I wish I had more time, but frankly, there's a lot you can get done part-time if you just go at it. My current biggest hurdle is neither time nor money nor realization of possibility... but because I do have limited time, I tend to overanalyze project ideas before starting them...