This is a great and confusing question, because of all large companies Google seems most inclined to want to own their whole software stack. Plus, they have an army of engineers who surely think finance software can't be too complicated!
So what gives? How could Google end up in this position?
They may have tried already. It turns out that financial software is really complicated. The basic functionality is never too complicated, but most of these systems are edge cases that needs to be treated specially. Also, a lot of behaviour has legal implications, so it would take a long time to develop software to compete with something like SAP.
Unless Google wants to get into the ERP software business, spending years developing such a system is a waste of money. Also, they'd need to maintain it for a very long time, and we all know what Google feels about keeping software maintained for long periods of time.
So what gives? How could Google end up in this position?