I think engineers like to create things. And they will tend, on the whole, to create new things when they have a chance, not because they want to justify their employment, but because they like to do it. And so, if you employ a lot of software engineers, you're going to have a lot of code. Combine that with an incentive structure (likely also created by engineers that like to make new things) which rewards making new things but doesn't particularly reward maintaining old things, and you'll have a lot of new things made, whether it's useful on the scale of the whole organization (something which is very hard to get a good perspective on as an individual contributer, anyway), or not.