Lack of easily filled gaps

You've got a point- the articles you can write without any specialised or local literature are mostly there on the few largest Wikipedias. Take en:Word, about as basic a concept as there is. The article is bad, barely sourced- though at least free of maintenance tags. That said, a quick web search is not going to vastly help you improve it, chances are you're going to need an academic library to make real progress. For our township in Pennsylvania, someone who lives nearby and has access to the local newspaper archives, museum, etc. could do something with it- those who are five states away can't- and very likely have no desire to do so. The amount of articles you can write from scratch, or make a run at GA+ quality, (that aren't recent topics) without touching physical sources are fairly low at this point.

I'm as opposed to systematic bias as anyone, but the obviously encyclopaedic articles, and those of broad interest to Western Culture are mostly at Start-class or above. What's left is the hard stuff, either making those articles shine, or going outside of the English-speaking world, and neither proposition is an easy one. As the work gets harder, it is natural those interested in getting it done are fewer in number; hard work being, after all, hard.