Why can't Canon make a 24-70mm f2.8 IS or better yet a 24-100mm f2.8 IS??

Obviously, Canon can make a 24-70/2.8 IS or a 24-100/2.8 IS. They choose not to.

WRT the 24-100, it's because f/2.8 and 4x zoom do not mix - at least with large full-frame image circle. f/2.8 and 3x appears to be the practical limit to design (3.25x for APS-C). To be as good and sharp as 24-70 it would be too large and expensive. Keeping cost in check then you get what Tamron tried in the '90s. They had a 28-105/2.8, but it was universally panned as too soft and too heavy.

WRT stabilization, I don't know. 24-70/2.8 is a workhorse for pros, and Canon/Nikon both seem to keep close relations with pros. You'd think if pros were clamoring for it they'd have added it to their new 24-70/2.8s but neither one did.

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Unapologetic Canon Apologist