Because most of the time it's something small and "dumb" that cause the issue, but with no human there it's not possible to fix it instantly. The difficulty is

Because most of the time it's something small and "dumb" that cause the issue, but with no human there it's not possible to fix it instantly. The difficulty is in planning for every contingency possible in advance, which you can't, and then hope that whichever one your mission encounters is in the list you planned for, essentially.