Which is probably way at the back of the developer's minds: the main audience of Linux is happy to compile from source (either themselves or via a downloaded package).

Exactly. Quite a few of the "problems" with Linux development ultimately stem from trying to shoehorn Windows-oriented expectations into Linux development/deployment. Very few applications in the Linux world target specific distros themselves; instead they publish their source code and leave the actual distro-specific finagling to distro/package maintainers (sometimes those maintainers are also developers, but they're regardless treated as separate concerns).

Of course, quite a few game developers are curiously averse to publishing any kind of source code, which tends to thus require compatibility efforts to happen on the developer side instead of the package maintainer side. This can still be worked around by targeting a specific distro (say, Ubuntu or SteamOS) and letting package maintainers for other distros apply whatever workarounds they deem necessary to get the app running outside of a "supported" environment (see also: Steam, Spotify, Slack, etc.).

I'll grab source code if I'm doing development or something obscure, but I feel like packages have been pretty solid and widely available since the early to mid 2000s. When working with source code I'll almost always install to a custom PREFIX--but if it's just an app the source is the last resort.

All of the commercial packages I've used at work for the past 15 years target either one or a handful of distros. Even the open source stuff that's either too new or outside of the default repos I much prefer to install from a package; turbovnc, grafana, chrome, etc. The few games I've seen have worked the same way as other commercial user-oriented apps.