Why? Well, some of it's solid code. That might be worth keeping. But some of it's technologies that fell out of fashion. And there's a bunch of stuff that people wrote in a frenzy, so there's a bunch of tech debt. And even if they stripped it down to the bones, there's a lot of upgrading to do just to get back to where they could do new work with modern libraries. From the business perspective it was easier just to glue together a bunch of outsourced services.
I'm sure those developers all meant well by picking cutting-edge tech. But as Dan McKinley says, we should generally choose boring technology [1]. As you describe, the costs of keeping up with interesting things is substantial. On rare occasions, it's really worth paying that cost. But, mostly, as with my acquaintance's company, the costs can accumulate while your attention is elsewhere, pushing code bases into a sort of technical bankruptcy.