Sometimes, the playback system has a limited amount of gain available...
Sure, but that could be addressed with better playback...
I understand what you're getting at but there are 2 other factors that come into play.
First is the cost factor...yeah I'd like to upgrade the playback systems in my apartment, in my car, better portable playback for walking around and in my boat and vacation house (actually I'd love to even have a boat and vacation house at all!
) My point is that upgrading this stuff isn't cheap. While it would be nice to never worry about normalizing it's something I do because otherwise I have to crank the volume higher than I want to (and induce more noise).
Second issue is uniformity....theoretically I could leave the volume knob set on each piece of gear and listen to various different shows and never adjust the volume because the levels will be fairly close. In reality this doesn't work exactly mostly because I listen to more studio recordings than live recordings. But at least I know about where to start with the volume knob for live shows vs. studio recordings.
You are right though, it could be addressed with more gain on the playback side. I look at normalizing like adding the cleanest and most uncolored possible gain to a recording. Hopefully my logic here makes some sense.