Why is the pressure sensor located lower instead of up higher in the boiler?

The sensor is designed to measure the pressure in the system. Placing it directly in front of the heating unit would provide false reading because of the internal components of the boiler that also have an impact on the size of the pressure (pump, elbows on tubes and so on…). The sensor is there to switch off the boiler when the system pressure falls below 0.7psi.