That "Reply To All" is not the only annoyance of how Outlook handles replies. Another example: if you open an email you send to someone and want to follow up with that person, you hit "Reply", and instead of putting the other person's email in To:, Outlook puts your own.
Gmail acts differently though and I got spoiled by both how it handles "Reply To All" and "Reply" to your own email. And it did make very much sense to me back then when I first noticed these nice user experience considerations put into Gmail.
Now to answer your question of why MS hasn't changed:
Microsoft has been notorious for not focusing on UX until it starts hurting their bottom line. They build (or acquire) apps and then just sell them through their creative sales force. That's been until recently when the competition in the software space became aggressive with other vendors on the scene.
Only with Windows 7 when Microsoft noticed those like Ubuntu stepping on their toes with lightweight OS's, they really focused on the speed and UX of their OS. SharePoint has been clunky all this time and only the recent versions saw improvements once cloud-based content repositories started to tap into their share.
When it comes to Outlook there is not competition (unless you move entirely to cloud based email like Google Apps), so Microsoft devoted only some effort to improving it (although I admin to love many of the improvements that came with Office 2007 and 2010).
It's also one of those things when something that's implemented ages ago doesn't change, until someone at some point takes a fresh look at it (just like Gmail did). So if a button says "Reply To All" and it used to serve to reply to "..All", no questions asked, then it will continue doing that.
However it should be a fairly simple logic to get done:
- If a "Reply To All" is hit, don't include myself. Quoted someone saying "behavior depends on Outlook not knowing your e-mail address" - this is simply not true: your email address is the one you have set up in your POP/IMAP configuration of Outlook, and the one which gets populated in the From: field.
- Similarly, if "Reply" is hit, and if there was only one addressee in the original email, instead of myself (again) include original addressee.
Those MVPs who answer the forums are technical people (and they are not the team who builds Outlook, but third-party Microsoft partners), and technical people are rarely good UX people.
P.S.: Microsoft did take a fresh look at email with the new Outlook.com, which is a lot smarter and has been built with very many fresh looks put into it (it's almost like Microsoft hired or contracted some of the Google engineers). Something even tells me Reply To All and Reply work properly there.