Electrical isolation problems when routing between pads of SMT resistors/caps
Most, not all, SMT components have insulator on the bottom, between the end caps, so the solder mask is not actually acting as a critical insulator and even if it wasn't there the trace would not contact any conductor. 

Of course, for manufacturability, it's generally better to have large clearances where possible, but there's nothing inherently wrong with running traces under parts so long as the design rules are satisfied.