Not all Android phones have a phone button. The G1 does, the Nexus One (which I have) doesn't. Mostly, there isn't really a standard for what an Android phone should look and operate like. I don't really expect this between manufacturers, but even within manufacturers there can be pretty significant differences between devices.

Also, because Android doesn't have a particularly strong stance on how UIs should work and should be laid out, there also hasn't been a very strong stance on how buttons on the phone should be laid out and what the device should look like. For instance, the back button can change functionality quite a bit between apps. Or also, the G1 has a camera button but the nexus one (foolishly) doesn't. Most devices have a UI completely unrelated to the phone hardware.

I wouldn't just pick on Android but I think it's a problem that Windows and Linux have had as well. The experience just isn't that cohesive and it makes them more difficult to use. I'm not really a fan of Apple, but it's definitely one place where they've put in a lot more work than others.