I was at 18F for almost 4 years. In that time, many of the projects I worked on were internally-facing. The US Government runs thousands of internally-facing websites and web applications. Many of them are entirely internal to single agencies, others are for inter-agency collaboration. Many of them require PIV or CAC cards for auth. And yes, many of them - whether card-authenticated or not - are utterly horrific.

This is a big part of why the USWDS project was created. 18F is tiny, and there's no way it could address even 1% of .gov by itself. Fortunately, there are many people all across government who want to improve UX on sites and apps. USWDS is a good starter kit that significantly reduces the cost of such projects for everyone, not just 18F.

^ what Yoz said (Hi Yoz!). Also within the last 6 months login.gov added support for PIV/CAC cards so while that product only handles the sign on experience, there is some hope for internal sites improving across the board. The tools exist at least.