The reason is that there are many, many, many nuances to consider when designing crypto systems, let alone ciphers, that many people who attempt homebrew crypto don’t take into consideration. This is why so many cryptographers say to 1) never design your own crypto and 2) never implement your own crypto. These ciphers and cryptographic protocols you here about were not the sole design of some genius mathematician and they were not created in a short period of time. These ciphers and cryptographic protocols are the collaborative efforts of many mathematicians and computer scientists working incredibly hard for months or perhaps evens years, and even then there are still flaws that are found in their work.

English below:

The reason is that there are many, many, many nuances to consider when designing crypto systems, let alone ciphers, that many people who attempt homebrew crypto don’t take into consideration. This is why so many cryptographers say to 1) never design your own crypto and 2) never implement your own crypto. These ciphers and cryptographic protocols you here about were not the sole design of some genius mathematician and they were not created in a short period of time. These ciphers and cryptographic protocols are the collaborative efforts of many mathematicians and computer scientists working incredibly hard for months or perhaps evens years, and even then there are still flaws that are found in their work.