This makes sense a lot of sense for virtual/downloadable products where you don’t need to manually approve the order. For physical products it’s really necessary for somebody to package the items and ship them.
WooCommerce is very flexible and I guess the team behind it assumed that physical primary products wold be sold so the orders are held with on-hold or with processing status unless somebody goes and changes that.
We at Orbisius are selling digital products & services so we don’t need to have somebody manually approve an order so we’re doing the autocompletion thing.
You may achieve this using a plugin OR you can paste the following one line of code into your functions.php file OR a custom plugin that you use to customize the sites you develop.
You can also try this Autocomplete WooCommerce Orders WordPress plugin