[rubinelli]

Assuming you are talking about web applications from banks and non-tech Fortune 500 in general, it's because many of them are:

  * incredibly bloated (and still lack most of the features
    that an actual human user would want)
  * poorly coded by armies of outsourced programmers
  * using over-engineered code built on top of obsolete frameworks
  * running on a "homologated" stack, which is often 3 to 7 years out-of-date

(I know because I was partially responsible for some of them, in my dark past.)