You are installing something that by it's nature requires a bit more understanding that just clicking on a .exe. Do you want to replace the default Ruby setup, which could potentially be used by something? Do you want to install new database servers? Do you want to modify Apache? Do you want to fiddle with SSH key gens? Do you want to have to delve in and change items of your .bash_profile? Do you trust an automated installer to be able to do it reliably on every single machine?
I'm not sure I'd have the confidence in something not going wrong and then sitting there with a half broken machine wondering what. At least this way at every step you can do the install > check & verify > confirm steps and make sure it's all business as normal.