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.
If you use autotest or watchr you aren't starting up the environment very frequently.