It just isn't compelling stealth. Aside from Act 5, there isn't one well-designed area that was meant for stealth. The game in general was made more as a third-person shooter than a typical stealth action game like MGS1-3. That was one of the most disappointing things about the game for me, ignoring the direction the story took. Easiest Big Boss run, too.
MGS2 is still uncontested when it comes to stealth, in my mind. Everything was made within the limits of what the AI could handle, and it makes for a very tough but satisfying experience.
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"When I was a young man, I had liberty, but I did not see it. I had time, but I did not know it."