For the same reason I smoke weed. They clearly love it and it has no adverse effects (just don't let them eat it, use it in toys).
I used to think that too. And I still smoke ( hash, which is different ).
However, it does make me complacent, which sometimes make me endure situations I would otherwise change.
Not to mention the toll on the lungs. Ask Willie Nelson if you'd like anecdata about how that can play out over a lifetime.
I've recently made the leap to edibles exclusively after a stint with a hot-air vaporizer (a la Volcano devices) and I now find the resin smell of smoking incredibly off-putting. The coughing, hacking, and lung stiffness that results from a bong rip even more so.
Doesn't lessen the complacency, but the delay your metabolism introduces can help you get started on projects before it sets in!