Flexe finds spare warehouse space for e-commerce merchants
bloomberg.com
bloomberg.com
Splitting inventory is an expensive activity. Often times you need to purchase more overall inventory and incur more warehouse receiving cost when you try and divvy up your stock around the country to decrease average shipping times.
They don't own their warehouses. This can be an issue if something goes wrong (eg they lose track of a large portion of inventory). Often warehouses hire temporary and low-trained workers. Things can go wrong if the warehouse doesn't have proper operational controls and can stay wrong for days (months even!) if a warehouse doesn't have a proper support team. Since they don't own the warehouses these issues are difficult to control for.
There's some competition in the space like Shipwire owned by Ingram Micro (who also lease space to Amazon). Also I believe you can use Fulfillment By Amazon (FBA) to fulfill customer orders even if an item isn't listed on Amazon.
This comparison is extremely misleading. Amazon either leases or outright owns their warehouse space and thus has total control over it. But Flexe's 25 million square feet are just listings; the property owner, not Flexe, has control over it, and if a better marketplace comes along they might choose to leave Flexe and list somewhere else.
There's also many buyers that make sense for the company, too. From "make it easier to build a business" things (like Stripe), to "we have a bunch of warehouses and could synergize with usage & market data".