THAT'S the reason? Well, don't be giving me credit then. I thought it was cause the bulkheads were holding up my entire flimsy, all siding, non-bricked upstairs.
Glad you typed that essay and not me. Phew.
Further to the original questions...
There are ways to completely delete the bulkheads in many homes with siding. Technically, I think it can be done in almost any home but as kemist points out in his first post, it may come at a cost - often undesirable vent locations, possibly even locations that shouldn't pass code (but probably get overlooked). And it can also get prohibitively complicated which would be another reason Mattamy, as a volume builder, would have to put the kibosh on some bulkhead deletions.
In our case 75% of our bulkheads were completely deleted - allowing us to install the cabinet crown moulding (to be done later). We left a small stretch over our patio sliders in order to avoid a negative impact on air flow/vent locations in our Master Bedroom - and I'll thank Greg for helping me with this solution

. Again, this bulkhead could also have been deleted...but the cost (undesirable vent locations) was more than I was comfortable with.