Because the first job of anybody who is responsible for green lighting game development at these huge publishers is to not get fired. Making a game that only just breaks even or even worse makes a loss puts you at risk of getting fired. Even a relatively small game from a large publisher costs a ton to develop and market and has increased risk that nobody will actually buy and play it, at least in the most profitable first few months.
Franchises are so popular with this crowd is because they do not have to worry about name recognition. Hardest thing about getting a brand new title out is just getting people to know it exists and then to be excited about it. Franchises you hardly have to to do any work for that, you know you are going to get press and gamer interest, they sell themselves right up until they release and people get the chance to see if its a house of cards or not.
Its that front loading of sales that they are after, the shops having to buy in stock, idiots who pre order or buy before its clear if the game is broken in someway. Its the most profitable time as the game is at its most expensive, and it enables rapid repayment of the development costs. Games that start slow and have a very long tail of sales do not interest them anywhere near as much as they have already moved onto the next project and already been judged on the initial (under) performance of the game.