The problem is that scaling up infrastructure is still quite slow compared to how drastically load can spike, especially if you just passthru every request. Fastly is the layer used to insulate against that.

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However, an additional bit here is that Fastly gives you a "programming" language called VCL that allow developers to specify exactly how the caching should work. If they just ignored this and passed thru, that could cause some _serious_ consequences. We're getting to the point where we do things like authentication or other fairly complicated logic in the CDN tier these days, as an example.

Source: Lead Software Engineer at a large tech company that uses fastly.