That's...totally not what the saying is about. It's creativity that increases with limitation, not quality. And then still: the limitation should be on the artistic element, not the technological one (if creativity is used in the technological field, it's never in favor of the end users).
I'd also remind you that the platform-specific part of creating a game is ever shrinking. Are you seriously arguing that quality increases when someone DOESN'T push unity's "compile for <console X>" button?
It's never that simple. You have developers, QAs and possibly BAs trying to implement what PMs ask for. Each platform has technical limitations to cater for. BAs and PMs rarely understand that concept. QAs work in then increased for each platform targeted. If a game is focused solely on PS4 for example, your whole QA team is dedicated to testing on PS4. If you bring in more systems, you have to perform TEs (Test Executions) on each platform. Granted, you probably only need to write test cases once. But there could still be differences that require different test cases to be implemented for different platforms.
As for development work, you are completely disregarding platform-specific APIs and OSes. For example, the Xbox 360 supported XNA as a framework for developing games in Visual Studio using C#. Stuff built in XNA wouldn't compile for PS3, as much of the code it depends on from the OS wasn't there (a slight oversimplification but you get the idea). The CPU architecture for both systems is mostly the same, but that doesn't mean games written for one are easily compiled for the other.
Interestingly, with the latest generation of consoles. its the other way around. PS4 supports XNA through mono and uses the BSD kernel. The XBone, despite XNA being Microsoft's creation, doesn't support XNA in any form.
It has admittedly been a while since I've used Unity, but I guarantee there are limitations to compiling for each platform. Targeting just one additional platform is a huge amount to take on, unless the game is incredibly simple. But even then, there's still all the QA work to consider.
TLDR: Targeting one platform for a game unifies the team towards a common goal and prevents neutering of ideas due to the limitations of that platform. The result being the potential for a game that can be as good as it can possibly for that system, rather than a compromise that works on multiple systems. The later may still be achieved but is far less likely than in the case of the former.