Unfortunately, high-res multimonitor setups are horrendously unstable, require specialized graphics cards (which usually sound like rocket jets), and have a mess of incompatible standards. I'd probably need to spend a week or two figuring out what graphics cards I need, upgrading my motherboard to have enough slots to take them, figuring out if there's a way to make it talk to my laptop, etc. Getting four 1080p displays working was enough work.
Basically, not a project I have time for.
Anytime that hardware becomes a project, it reaches 5% of the market. Most of the market are people who just want to get work done. That means word processors, IDEs, spreadsheets, PPT, etc. USB is fine even for multi-monitor 5k for normal work if you update the screen incrementally and downsample/compress video. Video games aren't necessary. 200 watt graphics cards aren't desired. Stability and ease-of-use are critical.
The trick would be making it Just Work. That means driver support (Linux, Vista, XP, MacOS, etc.). That means testing across a range of hardware and software. That means using a standard cable (USB or similar, not a pair of DisplayPort 1.3 with dual-link cables and a graphics card capable of virtual...). It also means performance testing for the stupid stuff (not issues of 60hz vs 10hz refresh, or 3d gaming, or even video -- just that there aren't funny issues where you wait 1 second for a screen refresh, or the system locks up for 30 seconds thinking).