Noel Borthwick [Cakewalk]
They deliberately dont add extra ports to keep them light and compact. You can always get a USB3 expander if you need more ports. Compared to any tablet/laptop hybrid I've owned the surface pro is the most robust and its frequently updated unlike those from some other companies. 

I tried an expander and it caused glitches with the interface.  When plugged in I could not record without issues.  Take the expander out of the equation and no issues.  
 
Though to be honest, this was in 2015 so it may be perfectly fine now, so much has changed since then, I might still have been on Windows 8. :)