You have to pick and choose.
If, every time you left the browser, it reset to the homepage, wouldn't you be pissed if you'd pressed home to check another app or do something else while you were in the middle of typing a post here or reading something that you're not sure how you found in the first place?
The phone's automatic memory/power saving system will close the browser completely after a certain period of inactivity (I'm not sure how long).
If you wait THAT long you'll see the homepage.
Bottom line, the homepage is for when you first launch the browser, but because the Droid multi-tasks, it can't be sure if you're trying to EXIT the browser, or just go take a look at something else on your phone and come right back to what you were doing in the browser.
You could also just open a new window and then press home. As long as you don't have new windows set to open in the background, you'll see your homepage when you open the browser again.
