Because to look after the site, you need to be up to date with everything that is happening in a specific environment. It is not enough to watch for a new update in the dashboard. After all, some plugins have been abandoned and are not being developed anymore – such plugins will never inform about a new update, and they may have vulnerabilities. You also have to keep track of security gaps, forms of attacks, etc. That is why we only deal with WordPress because we are experts in WordPress.