Pricing- manufacturing and parts is a small fraction of the total
cost, so the difference between the bulb and the housing plus bulb is
minimal.

They sell the whole unit cheaper than the (most expensive) part
because they move a lot of whole units. If they don't sell very many
replacement bulbs (and at this price they probably don't) then the
cost of inventory's much higher- they stock something that takes two
years to sell, that costs them something.

And if they didn't stock the replacement bulb people would complain-
What do you mean I can't buy a bulb, I have to buy a new one, throw
away the whole thing and reinstall it?