It's simple most large enterprises have large IT departments with Developers, Integrators, Systems Admin, Project Managers, Help Desk and all the other IT department Minnows you can think of.
They have integrated numerous systems over the years, from a vast array of suppliers to address all aspects of the business i.e. Supply Chain Management, ERP, HR, Inventory and a vast array of other business acronyms.
Most system architecture will be based around Java, MS, Oracle, IBM, and a complex array of other vendors.
When it comes to a web project, no Open Source Cart is going to satisfy their needs, as they need to integrate with all systems including the ones that monitor the toilet roll levels in the staff toilets. It is easier to build their own solution from scratch, mostly because they have the Resources, Money and Time.
When you're running your web shop from your spare bedroom you don't have this problem, your integration problems revolve around does your "Cart" accept CSV stock imports and can people pay for the items they want!
You don't have the TIME, MONEY & RESOURCE to do everything from scratch, so a ready rolled solution works well. If you actually manage to make a success from your spare bedroom business, you can then think of employing the vast array of IT bodies to squander large amounts of money to build your very own Shopping cart, and integrate it with all manner of other software of your choice.
Take it from me, a software developer who has assisted many companies in spending large amounts of money to re-invent the wheel albeit in different colours and features. Most of the carts available will do what you need, and more, but they all come with a learning curve (Time), and require additional plugins (money) to add great features that You (Resource) can implement.
Personally my favourites are Magento, WP Ecommerce & Woo Commerce. I have heard great things about OpenCart & ZenCart, but haven't used them yet.