Exactly, its uni. And it depends on the course coordinator. Personally, I think that some of the coordinators are just too lazy to make more assignments, and then I think that some assignments are under-valued (eg. contracts assignment was only worth 20%- you kinda dont even see the point in doing it). 30% upwards and then I take them seriously.
Also, some of those subjects whose final exam is worth 50-100% is due to the fact they cant really give you assignments. Like stat: they're not willing to make the group assignments worth so much because you might have "social loafed" (theres a psych term for you to remember for the exam Dave!) off the group and thus not have learnt much.
So I do understand the logic for some of them because they want to test your proper knowledge, rather than seeing how good you are at having 6 weeks to write an essay. It sucks at the moment I know, but once its over, its over. And really its not really that hard to put that amount of effort into something for a few weeks. And you dont want to fail: I know how much that feeling sucks, and how good the feeling is when you pass and dont have to do it again.
Just study honey!