The Cessna 172, and it's predecessors, the 150 and 152, are very common training aircraft for pilots, and have been for decades. The reasons for this are simple; the 172 could be considered the simplest, easiest, and most importantly, safest airplane a student can fly. They are easy to maintain, as parts are plentiful and downtimes minimal.

From Jane's Aircraft Recognition Guide:

"Started initially as a tricycle development of the Model 170 and went on to become the most successful general aviation aircraft. The high-wing Model 172 first flew on 12 June 1955 and remained in production for 30 years. Total build of all models: 41,000+."