A lot of companies spend massive amounts
of money writing software from the ground up.

This is because a lot of software simply
isn't available commercially. And if it
is available, you might not want to use it,
because writing your own may provide a
competitive advantage.

This is especially true in financial services.
Financial software isn't all simple beancounting,
which is why investment banks tend to be very
early adopters of high tech. Everything from
old Lisp machines to SGI visualization tools.
It's not all 3870 terminals and mainframes.

Next time you're in a Borders or Barnes & Noble,
look for a magazine called 'Wall Street & Technology'.

I would guess that the bank I work for has
significantly more non-mainframe programmers
than most shrinkwrap software companies.

These financial companies do buy shrinkwrap,
where it makes sense. In that case, it's
rarely modified. They also buy development
tools (components, objects, libraries) which
can aid their custom development projects.