In addition to the other answers here (e.g. already overvalued, liabilities, survivorship bias in perception, ecosystem factors favoring competitor):
What would be reasons NOT to invest in a small company that has great leadership, talented workers and an amazing product?
One reason would be because the market doesn't value that product. There are plenty of amazing products that have been produced by small companies with great leadership and talented workers, but the part of the world with sufficient exposure to the company just wasn't ready for those amazing products until after the small company ran out of cash. Investors in those companies lost money. Product-market fit or lack thereof is a key determinant of a company's future success.
Low barriers to entry could also dissuade an investment. Suppose the startup had a great product-market fit in addition to all the other plusses you mentioned. What would stop a much more well-resourced giant competitor from making the same thing? With larger volume and an existing sales/distribution network, what's to stop a larger competitor from selling the same thing for just a bit lower price and drive the startup out of business? There are some good answers to this question (e.g. patents) but if the startup doesn't have a good answer, it may not be a great investment.
Alternatively, high barriers to entry that the company has not overcome could dissuade an investment, unless you have reason to believe those barriers are not too high for the company. For example, if the company's product clearly infringes on a competitor's patent and the competitor has refused or not been asked for a license, that would be a red flag for investment.
Another good reason would be if you can't afford to lose that money. Investing in startups is highly risky. You should plan on not having access to that money for years, probably forever. Would not having those funds prevent you from being able to put food on the table and a roof over your head? Would it stop you from being able to pursue other goals that you hold as a higher priority (e.g. returning to school or having a kid)? Would it prevent you from having the resilience to weather stormy economic conditions for several months, during which time you may not have income? If the answer to any of those or similar questions is yes, then no matter how amazing the product (or even product-market fit), how great the leadership, or how talented the workers, you should not invest.
A bad capitalization table can also be a major turn-off. Even if everything else about the company is fantastic, if the existing ownership structure is sufficiently messed up (e.g. giving all sorts of conversion to control rights to early investors who do not fall into the category of "great leadership") then getting involved is a disaster waiting to happen. When these situations occur (and they do!), if the new investment is large enough to make a material difference in the company's prospects and there aren't other suckers willing to substitute with a similar investment, sometimes existing investors might be willing to renegotiate their terms, especially if they think the alternative (e.g. no investment and bankruptcy) would be worse for them. A small individual investor is unlikely to carry enough weight to incentivize this.