There are a few reasons for this. Most of the time the thermocouple that is used with the EGT gauges (K-type normally) are fully shielded, so their response time is fairly slow. You can get partially shielded or fully exposed thermocouples as well that will react faster, but at the cost of lifespan. Distance from the ports have an effect as well. Most people that I know of were putting them either at the end of the #3 or #4 port in the exhaust manifold, or right at the inlet of the turbine housing. For general monitoring there is nothing wrong with that. If you are doing serious tuning, then it needs to be pretty close to the head flange. Again, I think we all agree, a lot of people don't fully understand what they are looking at with EGT's.