Hey Ben, it looks like in the latest version of Unity 2018.2 and above they removed them from the install as they are building them. Here is what they mention:
https://blogs.unity3d.com/2018/07/10/2018-2-is-now-available/
Standard Assets replacement
We have removed the 5.0 version of standard assets from the Installer in 2018.2 and are working on replacement packages for each part of the former prototyping pack. The first package (Standard Assets: Characters) will be available as a Preview soon after the 2018.2 release. It will contain a First-Person and Third-Person Controller integrated with Cinemachine, as well as a prototyping environment built with Probuilder. For details, watch for a blog post on the subject in the coming weeks. For those of you who need the legacy 5.0 package, it will remain on the Asset store.
You can still download essentially the same standard assets package from the asset store here: https://assetstore.unity.com/packages/essentials/asset-packs/standard-assets-32351
