The reason why is that it’s an almost useless feature which gives people false assurances, always a bad thing:
1) It doesn’t work when the receiving email client does not have an automatic service that sends a reply. (Not only Entourage – there are many, many other other email clients which don’t reply.) Therefore, if you don’t get a reply you may incorrectly think it wasn’t received,

2) It doesn’t tell you that the recipient read the message anyway, only that his email client received it. Therefore when you get the reply you may incorrectly believe that he read it when he hasn’t.

So – bad both ways round. It’s a really feeble protocol, and good that Entourage does not (pretend to) implement it. The fact that Outlook implements it is an indication that the Outlook developers, unlike the Entourage developers, seem to think that everybody else in the world uses Outlook, which they don’t. The Entourage developers, being on a “minority” platform, know better, and are smarter in not implementing it. To be a bit fairer, the Outlook approach probably reflects the fact that, originally, back in Office 95 for PCs, Outlook was an extension of an earlier client that worked only as intranet – something you used only with other people on your own Exchange server. Outlook started life doing the same thing – there used to be two modes – one for “Workgroups” on an Exchange server, and another for “Internet”.

There were some poor decisions made when Outlook was extended to the Internet, that shouldn’t be there. This is one of them – it’s inaccurate and unreliable, since it assumes that everyone can do it, which they can’t. You’re much better off just not using it, since it cannot be relied upon.

And that’s why Entourage does not implement it.

— Paul Berkowitz MVP MacOffice