I'm not sure why you think that. Unless you use spork or something that preloads the rails environment then forks it on every test run, autotest/watchr will boot the whole rails environment from scratch on each run.
You're right. I'm distracted by the reported 20-30 second environment load time for each test when it only takes about 7 seconds for my tests to start with autotest in my current pretty large app on an older MBP.