The waste comes with asterisk

That "waste" (that overcommit turns into "not a waste") means you can do far less allocations, with overcommit you can just allocate a bunch of memory and use it gradually, instead of having to do malloc() every time you need a bit of memory and free() every time you get rid of it.

You'd also increase memory fragmentation that way possibly hitting performance.

It's also pretty much required for GCed languages to work sensibly