Cuz I don't want to
I just discharge everything and usually double cut at least the front lawn anyway so I essentially am mulching, just not in one pass. If I have a few heavy rows with "hay" on them I can always bag it quick too. Most of my accounts have onsite disposal areas.
I can't see being able to run a closed discharge on most of my lawns without a mulch kit very often, maybe in mid summer. Don't you get a winrow along the discharge side? That's the nice thing about the mulch kits: each blade gets it's own chamber so you get a nice even dispersal of the clippings. I haven't tried the Extreme mulching blades though, just the standard wavy mulchers and Gators.