It is not just money - it is respect, recognition etc too. And most engineers aren't good in socializing, being "known" etc whereas it is the exact opposite with sales and marketing (of course, there are exceptions - well known engineers with blogs, books, speaking engagements etc). I once worked for a megacorp - my manager was a terrific guy (this megacorp bought a smaller company and he came to megacorp via the acquisition), putting in 10-12 hours regularly, and if it weren't for him, the product wouldn't exist. But there were a couple of marketing guys who were getting paid a lot more than him, doing a lot less.

In every company wide meeting when they would do "recognition" the IT team would come at the end, if at all they remember. And this was a software company :(