That's because 'the norm' for web agencies is a site that does basically zero traffic. If a company hires a 'web agency' that's by definition because the company's business model does not revolve around either a web property or app.
Whether that's a gas station company or a charity or whatever, the website is not key to their business success and won't be used by most customers apart from incidentally.
With that in mind most agencies know only how to implement a CMS and simple deployment perhaps using Cloudflare or a similar automated traffic handling system. They don't know anything about actual infrastructure that's capable of handling traffic, and why would they?
A lot of agencies are 100% nontechnical (i.e. purely designers) and use a MSP to configure their desktop environment and backups and a hosting agency to manage their deployed sites.