why do people offer seo services

i had one basic question.first of all dont flame me for putting up this ques.but was curious i see people offer seo services here and some offer them for really cheap and yes they know what they are doing thats y they are offering services. surely these guys must be good at keyword research also and not only making those crazy link pyramids and structures. they offer seo service at 100 dollars, specially that 12 insane link pushes at 97 dollars or so or so (some are way cheap than this).i mean if they are doing this for someone for a one time payment why not do this for themselves and reap monthly benefits since most have a good team..i cant really understand the whole idea. lastly dont flame me guys

They do it for themselves as well, but offering their services allows them to make additional money. They can do as much SEO on their sites as they please, so they do not need to worry about a customer that they offer their services to outranking them.

its also a more reliable source of income. if you're really good at one particular type of link building (i.e. private networks, .edu) and you've perfected that method, then its worth it to sell it because its easier for you to do than others (specialization).