Because forums and Q&A portals are exactly that. Forums for discussion are for people to discuss problems, theories and solutions - not for SEO'ing your clients whilst attempting to stay relevant to the forum.
If you come to this forum and post some random question followed by a link drop for your client then you WILL get banned. This really goes for all forums, we dont want the threads to be SEO targets - we want discussion!
Also as a MAJOR point to make is that this forum is NO FOLLOW as are many others so the effect of SEO'ing in here is minimal and will result in your account being suspended.
(I am not saying that that is what you are doing, just giving an example!)