Might be financially a good idea - perhaps we are just lazy and leaving money on table? Hard to say.
But it is such a pain keeping track of who's got what features enabled, what the pricing structure is, more complex invoicing, customers are paying already etc.
We would for a new major "product" but we do not for smaller features.
Why not sell it as an upgrade/add-on/DLC then? More money for you and less hassle and bugs for the customer to deal with.