Editorial Board Listings without subject descriptions
You hit on the reason, it's the amount of work necessary to maintain an up-to-date description. Classifying research is difficult because of how specialized knowledge is these days, e.g. an example I encountered recently is two people at a social function discovering that they are both physicists, and furthermore, both submit papers to arXiv that are classed as high energy physics. This makes it sound like their work is closely related, but then it turns out that one person works on lattice quantum chromodynamics and the other works on landscape cosmology, and they have zero chance of understanding each other's papers. The journal editor will typically not know how to classify the editor. For example, for the two topics above (lattice QCD and landscape cosmology) a typical journal editor might recognize them as physics, but I would be pleasantly surprised if an editor I manage can tell both are in high energy physics.
Since the journal staff are not very capable of classifying the editorial board's subject expertise, it becomes very hard for the journal to maintain an up-to-date description. Authors are experts on their own work and therefore the best-positioned to tell who will be able to handle their paper.
That said, many journals are trying to make it easier for authors. If you get the chance to choose subject codes for your manuscript, that would be a relatively easy way to find the editor most suited to your manuscript (although you might still end up with an editor from the other end of the topic).