Since the coin collecting boom of the 1960's, many world mints started producing special proof or mint sets or individual coins for sale to collectors. Because of the marketing behind this, there are many thousands, sometimes millions, of mint condition coins in existence. These special collectors issues are often packaged, so there is every likelihood that the coins will be retained in their packaging, and thus will remain in mint condition for many years, and seldom get accidentally spent. This means that there is more than a ready supply of mint condition modern coins, further reducing the chance that any coin found incirculation will be valuable. It may seem obvious, but coins are made of metal, and are made to last a long time, and withstand a lot of wear and tear.
Coins are one of the most durable manufactured artefacts produced by civilisation.
Mass Produced Coins are also mass produced in vast quantities. For example, since 1971, over 12 billion pennies have been produced. Another perspective is to consider that as over 200 pennies per head of population. One partial answer is that many small coins become lost. Taking the lowest mintage figure for any mosern base metal 2 coin, the 1,443,116 issued in 1994, the number of coins collectors would need to increase by somewhere between 10 and 100 times the current number.