Why not? We're talking about a galaxy wide rebellion and you have a problem with them managing to assemble a starfighter factory.
Yes. The Rebellion was an insurgency. Their strength was mobility and distributed organization. They had to be constantly on the move from one base of operations to the next and divided into cells to avoid Imperial capture. A factory is nothing but a great big target that they can't protect. Having a vital logistical resource that can't be moved would be a huge handicap that they couldn't afford. That's why rebellions, insurgencies, resistance movements and terrorists always use scrounged together, out dated, surplus supplies. They don't have the infrastructure to make their own equipment, they have to steal it.
Trust me there have been rebellions throughout history that could manufacture equipment and hiding a starfighter factory in a setting like Star Wars would be much, much easier then hiding any kind of factory or shipyard on Earth.
Edit: It is also possible theoretically for the rebellion to create mobile factories for fighters and smaller equipment by installing the manufacturing hardware in a large freighter. I don't know that they did that but they might have.
Edited by RogueCorona