I said ship, not create. It takes very little effort to do that (copy some files), but for an even easier solution, they can link a toolkit in statically (see fltk, a toolkit meant for that).
Your last sentence just proved another point from that list - Linux has no good stable APIs for GUI development. Which means every ISV, according to your own words, have to create a toolkit just to run their software on Linux. Big companies can surely perform this feat. However a lot of small development teams and independent software developers (the ones who develop miscellaneous utilities and games) aren't in position to do so.