The short answer is that they deliver solutions and support. What they sell, primarily, is the ability for someone at your company to call them and say "we have problem X" and they will respond "no problem, we can solve problem X for you", and they will take care of the rest. This usually costs millions of dollars.
Let's say you are a large Fortune 500 company. A bank perhaps, a large retailer or an oil company. Your net worth rivals Amazon. You do not use AWS or GCP, you host your own data centers, and you have for decades. Now let's say you need another few petabytes of storage with global replication, durability, availability, you need hardware delivered, fibre cables routed, software to tie it all together and someone to call when you run into problems or someone on your backend application team can't figure it out. Or maybe you need Oracle and SAP integrated with Active Directory for your 100,000 employees.
Well, these companies will make that happen. Or many other problems you might have, such as building custom applications, implementing tooling, designing systems, whatever. It's kind of like asking "who employs all these personal chefs?! I've been eating for years and I've never needed a personal chef. I just go to the store, buy some groceries, and make food from the recipes I find online!" The thing is that some people just don't want to deal with all that, and they have enough money to make it someone else's problem who has done it before.