Many apps do this

Yeah its pretty common for many apps to close and remain running in the background, especially apps that have a reason to run in the background. For instance other software with the same behaviour

Discord
EA App
Battle.net
etc

Pretty much any app that deals with messaging and communication its standard behavior and not a bug.

引用自 BeatsByDad：

Why would the X in the corner not close steam? the X minimizing to system tray is one of the most stupid things i've ever seen. When i press the X on something it should close completely... This has been a problem for 10+ years, why hasn't this been fixed yet?

There is nothing to fix as Steam runs as service. X minimises it to the system tray.

To exit and shutdown Steam.

Menu > Steam > Exit

Or

Right click tray icon > Exit.

It is standard behavior for anything that runs as a service to not close but to minimize to the tray.

Steam
VPNs
Logitech support software
Skype used to

...and so on.

How did it take you 10+ years to figure out that some applications run in the background, even if their GUI component is closed?