It's frowned upon by people who need high availability with minimum complexity of hardware and software configuration. There's very little state one can reason

It's frowned upon by people who need high availability with minimum complexity of hardware and software configuration. There's very little state one can reasonably store on a single web server without interrupting sessions if the server fails, and there's almost none you can store if you can't guarantee user X will always connect to server X for the duration of the session. Not everyone must solve these problems, but they quickly come to the forefront for anything beyond rather low-traffic sites.