Well, total compensation, say at Google, can be around 200k for senior guys. A lot of people in finance are making less than that and so people do leave for software jobs.
As I have said before, really huge compensation levels in finance exist for VP-level people. Please keep in mind that "VP" in finance means something like "pretty senior", he might no be a manager at all. These guys have around 5 years experience and if all went well for them they can be making around 300k. These would be hard to match en masse while avoiding a situation when of two engineers doing the same job one is paid X and another 2X.
Hedge fund managers make orders of magnitude more money than their employees. Investment banks work much like law firms, if you are a partner or partner track you can make tons, but frequently that is built on the back of average paid employees.
What would tech company compensation look like if we included Brin/Cook/Zuckerberg?
There are lots and lots of small shops out there that have good but not idiotic compensation packages. The tech companies could (should?) match them if it weren't an ingrained part of the culture not to.