Precisely because it is pulsed. A typical volt meter will just show typically 1/2 the battery voltage or whatever dwell cycle is on the system. To get an RPM reading, you need to read the frequency of the pulses (i.e. how fast they are being generated) not the voltage or current. An analog tach (as pictured in the above schematic) integrates the pulses over time and produces a current that is proportional to the RPM, a full description on my web page:
http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml#Theory

A digital tach likely reads the time between pulses and inverts that period to calculate revs per unit time.