They do, subtly. It's only major overhauls that are rare. However, I think they'll be working on mobile apps a fair bit and the subtleties of interaction, since Google+ has grown in leaps and bounds through major design overhauls (both the new app and web interface are very good looking and good to use) and flashy ad campaigns. I guess they're trying to pre-empt the effect the relatively good-looking Facebook had on disgruntled MySpace users jumping ship from over-customised profiles.