Speaking as someone who has worked at multiple such companies, they don't buy off the shelf because you can't get a big budget and build a big team and get a big win and a big promotion that way. They also have so many teams they literally invent the same thing 3 times over. Sometimes building and firing the same team 6 times over as they try and fail to build one thing.

When a company has more money than sense, it enjoys spending it more on building rather than buying. It's also much easier to justify dedicating FTEs to yet another internal project rather than vetting a vendor and churning out a PO for a product you don't know how long you'll use.