A static site with no backend still needs a backend
The moment a static site grows a contact form, it needs a server to receive the submission - which is usually the point where a project acquires a framework, a host and a deployment pipeline it never otherwise wanted.
A form endpoint is the smaller answer: one URL to point the form at, submissions collected and forwarded to you, and the site itself still a folder of files on a CDN.
Knowing before your visitors do
Most outages on small sites are found by a customer, because nobody else was looking. The failures are rarely dramatic - a certificate lapses, a DNS record is edited, a deploy ships a page that returns 500 to everyone except the person whose browser cached it.
A check running from outside on a schedule turns all of those into an email you get within minutes, which is usually the difference between a short incident and a long one.