Firstly, because the whole thing started with good intentions to get some money in advance in order to be able to finish a product, but then some companies realized that it's a good idea to pre-sell games and then deliver half-assed product. And it's not "few bad apples", lot of developers/publishers (even AAA) do that consistently, I decided to not support the practice.
Secondly, when you consider it business wise, if you do not have money to finish a project and I decide to throw in some money, it's not a purchase anymore, it's an investment - completely different expectations and I can negotiate revenue percentage for example.