They may wish to discuss the terms of the offer with you because some aspects of it are complex. They may be expecting to negotiate an offer with you, which is faster face to face than it is via email. It may also be that back when email was less common (or even didn't really exist outside of education and very large corps at all) someone wrote down in the HR manual that This Is How Things Were To Be Done and the reasons have been lost in the mists of time.
It sounds like you're rather resentful of this. I'm not sure why but it should be a good thing to talk over an offer with a potential new employer. It's a chance to make sure you're absolutely clear on the terms of the offer and that any queries or questions you have will be answered. "Arm twisting" also goes both ways, remember, though if you see the relationship in those terms before even signing up to it then you may wish to continue your search for employment elsewhere.
You don't have to accept terms you dislike simply because they're offered verbally. You don't have to accept anything until its agreed on and handed to you as a written contract. The last few job offers I had were handled by telephone. Not quite face to face but still very immediate, and this wasn't a problem. I made my points, they made theirs and then we met somewhere in the middle.