Sigh. It's not just a fricking driver. It's an entire framebuffer you plug into a USB or Thunderbolt port. That's why they are more expensive, and why they even need a driver.
A 1080p monitor has one quarter of the pixels of a 4K monitor. The necessary bandwidth increases with the pixels required. Apple chooses instead to use the bandwidth they have to support 2 5K and 6K monitors, instead of supporting say 8 or 10 1080p monitors. That's a design decision that they probably thought made sense for the product they wanted to produce. Honestly I agree with them for the most part. Most people don't run 8 monitors, very few have even 3, and those that do can just buy the higher end model or get an adapter like you did. If you are the kind of person to use 3 monitors you probably also want the extra performance.