The point is that crypto is hard.

Therefore many people need to check it, to ensure it is correct.

The crypto you rolled yourself is unlikely to have been extensively tested.

It's like alpha software. It may have no known bugs, but it hasn't been tested in the wild. Once it has been tested and the bugs fixed, because there will be bugs, you can release a stable version.

You shouldn't rely on alpha software for critical things, like keeping secrets, secret.

So yes, write a crypto algorithm, but don't represent it as working correctly if you don't know it is or force others to use it, and don't use crypto that hasn't been extensively tested.