You can do rapid development with Java--it's less a condemnation of the technology and more the process that happens in big companies.
I've worked on contract in large and small companies, and the amount of paperwork to go through in large companies is ASTOUNDING. For example, we once had to get a test account created in the test domain (not even production). It took 23 emails, 4 IT requests, and 6 hours to complete--and that was the "accelerated" path.
But bureaucracy is only part of it. The real reason it takes a company to build something so freaking long is testing. Not just test-driven development (that's crazy talk, by the way), we're talking testers. Lots of them. Now, I'm all for testing (yay testing), but companies go to the extreme. I've had more than one of my projects take three to four times as long to test as they took to develop. Sometimes you luck out and get really good testers, though, so then life is peachy.
But to answer your question about why companies use Java, it's because it's widely taught, and programmers are thought of as interchangeable pieces. They can bring over a ton of people from out of the country for cheap labor, or just hire a bunch of D+ CS students and put them in cubes and give them detailed specs and just let them go at it. There's no thought to it--they're cogs in a machine.
Also, sometimes technology choices are just at the whim of people higher up--they have a favorite technology and push that as "the way," even in situations where it doesn't quite fit. Such is life.
To sum up, companies are slow to develop because of overhead and bureaucracy, and they use things like Java because they want cheap programmers, not good ones.
Edit: I should clarify something that came off as really presumptuous and pompous. Java isn't a bad technology, per se, and there are plenty of great programmers who use it. My point was that Java developers are easy to find cheap, so that puts a vote in the win column for larger enterprises.
That being said, there are a lot of really good reasons to use Java, and if you know Java really well, you can put together a CMS as fast (if not faster) than somebody slinging PHP or Python or whatever their language of choice is.
My main goal was to address why enterprise development is often considered slow by startup standards, and what causes that. I know a few really great Java programmers in big enterprises, and I know they often get frustrated by the glacial pace of enterprise development.