Because it's not only a matter of salary, you have to put "all in the balance" when considering a potential move. Personally, I was giving a lot of value to the mobility within the company. I started as a computer scientist in a bank and became a trader a couple of years later. Outside of my company, it would not have been possible because I didn't study finance.
Was it hard to convince them to give you that chance? Some companies seem to prefer to hire from the outside rather than to give the chance to someone internally.
I joined the desk as a computer scientist. They quickly gave me the opportunity to develop the strategies with them and thus to understand/know the business.
At the bank for which I was working, this kind of move is not uncommon.