The other explanation is that they don't really want users to have most browser extensions. The browser extensions either become features that google wants to embed in the browser, or things they don't want, for business reasons. In either case it is better if the extension dies after a year or so.
BTW, this doesn't have to be a conscious choice of anyone at Google, it could just be the way the incentives turn out.
BTW, this doesn't have to be a conscious choice of anyone at Google, it could just be the way the incentives turn out.