Why doesn't NASA just again build the Altair LM or a modified Apollo LM for the Artemis project?

Apart from the $12B development cost, Altair required Ares V to launch it and its Earth Departure Stage, without which it would never have gotten close to the moon. SLS doesn't have the capability to launch it. Even Ares V wasn't enough for everything, which is why they came up with Ares I and spent years and billions of dollars trying to figure out how to launch people on top of a Shuttle-derived SRB without killing them.

The Apollo LM was designed and built half a century ago. Its plans are all microfilm and physical blueprints, the people who understood them are largely dead of old age, the materials and construction methods are decades out of date, and most of the companies that supplied parts and did the construction are long gone. It would not be a shortcut to anything, and would be useless without the CSM...Orion and its service module would not be sufficient to deliver it to the moon.

Both Altair and the Apollo LM were two-stage vehicles, with a small ascent stage that leaves the majority of the vehicle abandoned on the surface. That's the only component that could even theoretically be reused.