... Why can't there be one or five cryptocurrencies that all projects can use?
it is simply because there is nothing stopping you from copying the code of some altcoin and create
money out of thin air with near 0 effort and that attracts thousands of people that are trying this every day. the number is a lot more than 1600 but most of them die in infancy since they are so shit! and nowadays with ICOs and the easiness of creating a token the problems have doubled.