Because HSS actually cuts the metal, even at low speeds and carbide actually plastisizes the metal in front of the cutter. You can not cut metal at low speeds with carbide and get a very good finish. Carbide needs to be run way above the surface speeds HSS can be used at. Plus, when carbide chips or cracks, it can't be sharpened. You just lost a very expensive insert.
The average guy, working in his home shop, will never be able to use the advantage carbide will give him . HSS will do it all.
Bob
Sorry Bob, but that carbide information is pure Puckey, of the bovine variety.
If you buy negative rake, coated inserts, and avoid all the others, it's still only true in some of the cases.
There are carbide tool inserts available in about every edge type you can imagine, then some. Sharp edges are not uncommon any more.
Spend some time in a Carbide tool supplier's catalog and try some of the varieties out, before you repeat the old information. It was true 25 years ago, but it isn't anymore.
Put a couple tools holding CCGT inserts into your repertoire, Maybe some VCGT, if you use the 35 degree diamond inserts.
They cut just fine at slow speeds too.And leave a nice surface finish, provided that one is not trying to make one's project out of rebar or similar quality bar.
The hardest thing I have seen for folks to get used to, running carbide, is bumping the tip or edge of the tool against the work when it is not turning. Stopping the spindle with the cutter in contact comes a close second place for ruining inserts.
I like HSS tooling, but would rather buy six pounds of blanks than pay for HSS pre-ground inserts.
For the most part, a guy that can blow the end off a carbide insert, can do the same with a HSS one, and the ability to resharpen it, really does not come into play in crashes. Guys that cannot grind their own tools from cheap blanks, will not be able to sharpen the inserts anyway.
Those kits, aside from their prices, are still pretty decent, but the FNG metal cutter that looks at them as his or her salvation from the terror of having to learn how to grind HSS blanks, is going to spend a bunch of money and end up unhappy.
The average home shop guy can do well to have a few carbide tools around, and will get good use out of them. They need to educate themselves, bite the bullet, and buy quality tooling and inserts, though, or it's money wasted.
Cheers
Trev