Cesium is a 3D viewing environment. It doesn’t solve the data issue. It was developed an open source replacement for Google Earth Enterprise customers. A single seat of GEE used to cost roughly $50,000 and came with no data. You read that right. No imagery, no vectors, nothing. All GEE customers had very large expensive data sets of their own which they visualized in the app. Cesium happens to easily connect to legacy GEE servers and google no longer has to maintain that platform for enterprise level customers. Instead they can focus on their main user base which are regular GE and Maps users.
SketchUp’s Add Location feature is powered by 2D map tile service which is more like Google Maps than Google Earth. Trimble actually has its own tile serving infrastructure and teams which maintain those services. But again, the difficult part is sourcing the imagery/data. There are open source imagery sets available to Cesium users but none of them are anywhere close to quality of DigitalGlobe. There are probably open source imagery sets out there which are higher resolution but they cover small areas and are quickly out dated. SketchUp needs a global solution that is updated on a regular basis.
As DerekEdison mentioned, PlaceMaker delivers the same DigitalGlobe imagery as SketchUp Pro but they have a monthly quota of 250 tiles per user. A tile is a 1280x1280 [EDITED] pixel image at some zoom level. One of the nice features of PlaceMaker is that they allow users to grab large areas at the highest resolution. However, if they didn’t have a quota, they’d go broke. SketchUp doesn’t have the ability to enforce a quota right now which is what limits the kinds of imagery products we can offer.