What are the reasons why we do not have VTOL commercial airliners? I understand that there are noise issues, and that there may be an economic case dependent on stage length, but other than that I'm not quite sure why they haven't happened. Is there something technical I'm missing?

Exhaust gas re-ingestion doesn't seem to be that great an issue, it looks like you just install fences to make sure that exhaust can't get into the inlets.
Commercial airliners are significantly larger, and therefore heavier, than aircraft with use V/STOL. Because of that, it would take more fuel and engine power to run that airplane, driving up the cost of running it. There are also many other airframe stress and balance factors that come into play when doing VTOL.
If you are including turbo-prop airliners in your definition of airliners, they could do V/STOL with a configuration like the V-22 Osprey because they are small enough.