This is because this is an older builder that was created before mobile sites were a common feature. Mobile sites really began back in 2007 when the iPhone came out and didn't really get popular or really usable for many years after that. The site you are referring to was originally built back in 2011, and at that time, only about 10% of internet traffic was from mobile and being "mobile responsive" hadn't really taken hold. That builder unfortunately just wasn't built to be compatible. For some time, we offered mobile friendly sites by using a 3rd party company, but unfortunately that company no longer provides that service. You can definitely rebuild your site in WebsiteBuilder, which does provide that feature.