A few reasons why authors need a support system:
The work of an author is most often a solitary one. Extreme isolation is not healthy for any human.
There are a million techniques to learn and keep track of while writing. It’s always good to have support from beta readers to catch errors.
Rejections can be rough and it doesn’t matter if you have “the best book ever written” its going to get rejected at least once, or a hundred, or a thousand times.
It is easy for ‘the creative muses” to take over you life. You can’t forget about family, bills, exercise, etc. It’s good to have someone there to keep you grounded.
Writing a book is like running a marathon.
It takes a lot of prep before you even start the race. There are hills and rough spots you have to fight through. Often there are times you just want to quit. That’s why authors need support systems standing on the side lines to cheer and encourage when things get rough.