That's mostly because a real lightsaber that cuts metal and stone is ridiculously useful. We had a DM that made the mistake of giving us one in 2e. It worked exactly like a lightsaber... for about three sessions. Then it basically functioned like a brilliant energy weapon. A real lightsaber is too useful.
Adamantine weapons do pretty much the same thing. Of course, they're also extremely rare.
I remember back in our 3.5e days our DM would have similar trouble with our magic-heavy group using Etherealness to bypass barriers. It got to the point where pretty much any secure structure we encountered would have its walls laced with bricks made of pure force.