Because buying new stuff is wasteful and only a temporary solution to the problem?
Look, I like nice, fast machine as much as the rest of the crowd here, but let's not kid ourselves. In a years time the software we run on it will have become even more bloated and things will be just as slow as they were. And then a new machine comes out we all want to buy that.
There will be some ecstatic blog posts again. Which is weird, I think. Needing a faster machine for you daily work should feel like defeat for a developer.
If you need a new laptop and can afford it, by all means go ahead and indulge. In the mean time, I am typing this from a 7 year old Latitude that I use every day for software development and that shows no sign of aging, apart from some scratches. And each time I read a post like this I wonder: what do these people _do_ with these machines?