Perhaps it's a regional thing then, in my country 2x4's aren't exactly cheap if you're buying enough to build an e.g. woodworking bench, and you need a planer even if they're pre-planed.

I'm currently planning a studio desk right now, funny enough. I could buy one for maybe €500, but just the heavy duty shelf sliders I need will be €70 for a pair. A pre-bought one will also have a light aluminium frame, whereas mine is going to weigh a ton with steel tubing.

Like you say though, if I make one it will fit the space perfectly etc. All in all it's worth it for me even if I personally don't save money