Even in HO and N building kits have become a smaller market, ready to go has become a big part of the market, time being a big factor from what I understand.
In terms of why they don't build the same thing in O, the answer is the cost. Tooling for an O scale structure is different than HO or N so most of the cost of creating an O scale structure is absorbed by O gauge sales. O gauge is dwarfed in size by HO and N scale market. So even assuming the costs to make an HO scale and O scale kit is the same, in HO it might be spread over several thousand units sold, in O it might be 100.If the production cost was 100,000 dollars, for HO that would be 50/unit@2000 units. For O it would be 1k/unit@100 units. Obviously this is a hypothetical, but it shows the impact of size of market ( and yes, it could be that O would sell 200 units, so would be 500 per unit).
People are assuming that O is this big market, it isn't. Not to mention that a lot of ppl prefer rtr.
One other note,I think when people talk about kits, they aren't talking craftsmen kits,which some if the manufacturers in that list are,small volume mom and pop kits that are $$$, they mean more mass produced kits a Lionel,mth,etc would come out with.
In terms of modern buildings I think a lot of that is because a lot of ppl in the hobby tend to model older eras, especially the transition era that saw both diesel and steam. This could be changing as newer modelers who grew up in the modern era want to model current railroading, but when I look at HO and N my impression is that it is the same way, lot more older structures that modern ones as a ratio.
One of the other reasons for older structures is that they are using tooling that has been around a while to produce them. A modern building kit would be new tooling cost and risk they can't make back the cost,whereas an old reliable has been paid for and depreciated, so cost of tooling likely will be little at this point.
So the size of the O gauge market and the trend towards rtr in general is going to mean a relative lack of kits,especially new ones.