While what you say is true, it's been my experience that many times a functional product is more important than it's validation. Sometimes a product has to mature a bit (and even find out if it has a potential customer base). I'll once again use Ventrian's modules which I rate as one of the "Must-have" subscriptions for quality modules that get the job done (SimpleGallery, PropertyAgent, NewsArticles, etc), but they do not always pass the mustard for validation. This has not been a huge deal for my clients, as they only really care if the module "works" across the big 3.5 browsers (IE, FF, Opera, Safari). Now, "works" is a very general term. For what the customer wants, they can add, change and delete content all day long with no browser crashes or funky stylesheet explosions... other than that he/she doesn't much care.
Now that's not to say that I don't care, but I have to be prudent with my time and resources, so I participate in the forums (DNN, Ventrian, XMod, etc.) and will occaisonally spout the "please more valid front-end code" plea but I also understand that like me, the developers have only a limited amount of time and resources as well. Now compound that with the fact that DNN is a constantly evolving framework and what works today may not work tomorrow, and that makes the dev's job even more tedious and creates a decision of New Features v. reworking the core module code, and most of the time Features win since customers pay for features and updates.
The only other catch I can see is that there are 20 bazillion ways to design a skin from pure Table hate to CSS beauty and everything in between, which I have no doubt makes the issue of creating a highly functional, multi-featured module that passes validation a nightmare.
Now alot of this is based on assumption as I only deal with the design / coding / skinning of sites and not the actual programming of modules so I can't say I'm the end-all-be-all of DNN information but I can only imagine what the devs are having to work with considering where DNN came from and where it needs to go. Luckily we have a huge, talented and capable community and we just need to support our devs and give them as much constructive feedback as they can process.
ok, getting off my soapbox...
SB