Some people want immediate responses and hope responders will send replies via email. We typically discourage this since it doesn't help anyone else. We like a record so that others can benefit from the solution too.
It's nothing special about this site. Throughout the internet, many people avoid posting their email address publicly because that can make it more likely that you may receive spam from someone. Typically spammers use programs to find e-mail addresses autmatically, and firstname.lastname@example.org is easily recognizable as an e-mail address. By writing abc at somecompany dot com, it becomes harder for spammer to recognize the e-mail address programmatically - while most humans with any common sense can still figure out what the address really is.