It's funny I have the exact same instinct / reaction as you do (and I certainly would do that if it were my company I were building). But I think we're probably both naive.
Some thoughts:
* Most companies are not software companies, and most c-level managers are not tech leaders. They treat IT as an annoying expense, like taxes. The idea of hiring software engineers to gather requirements and run a lean process to build efficient software from the ground up on a modern web stack is as foreign to them as setting up a chemical processing pipeline and logistics management network is to me.
* Rolling your own solution takes time, and more importantly, energy and involvement of the whole company. I would argue that it might take less time than a big SAP implementation, but with the latter you can just hire a big consulting firm (there's that cost center again...) and they'll deal with everything for you so your people can just work.
* This is a bit of a chicken and egg thing, but: A lot of SAP's customers aren't startups. If you're hired in the be the CEO of an oil company running major pipelines, you've already got SAP running everywhere. And you do your first big move and acquire a small competitor, and THEY have SAP running too. So why are you going to embark on a huge software development project when a) you're not a software company, b) you're not a startup leader, etc. etc. etc.
Although to be honest, the success (?) the US Government has had doing basically what you propose via the USDC / 18F does give me some hope. But alas, much of the world is too far gone already (sigh).