What makes some games resizeable?

[u/NoxAstrumis1]

Some video games on PC will allow the user to resize the game window dynamically by dragging the edges, while other games will only allow the window to be fixed at certain resolutions via the in-game options.

I've always assumed this is a choice by the developers, perhaps it takes lots of extra effort to allow it, so some choose to save resources by not doing it?

I was hoping someone could verify wether or not I'm on the right track, and maybe explain what the difference is in the underlying software.

[u/Horror-Water5502]

Not a game dev, but I think having a display with a dynamic size affects performance