But that was exactly my point; Google knows about trees, and they have 3D models of them. Not everywhere in the world, but at least in the areas that Sunroof is covering now (and many others - like I just checked my parents' house in Wisconsin). As to your "most people don't live in urban areas" comment, the US Census Bureau would say otherwise. According to them, it's about an 80%/20% split between urban and rural.
As far as SolarCity, they put exactly what they can charge you in their contract. If someone didn't read the contract to find out what they'd be paying, that's their own damn fault. Personally, I got my solar panels from them, but I bought them outright - no lease, loan, or power purchase agreement. They still warrant both functionality and energy production (with a per-kWh payout if they're too low) for 30 years, but I owe them nothing. That said, the sales guy I dealt with was a bit of an asshat, trying to push me with high-pressure tactics. In the end, they were still the best deal, both in terms of price and future maintenance, and everybody besides that one sales guy was very nice.
Solar certainly doesn't make financial sense for everybody. And you can't put a solar system on every roof today and expect the grid to still function (see: Hawaii, where many places are not allowed to add new solar systems). However, California (for example) is mandating that utilities build a lot of energy storage, to help smooth out the burst of solar power available, so more people can install it. A friend of mine works for a company that provides such storage, in the form of gigantic flywheels.
In any case, for many people, solar is a good idea. Google's tool helps you determine what makes financial sense in your particular situation. And its estimates (compared to what I actually paid and what my panels actually produce) are more accurate than many of the other estimates I saw while shopping. And even if it's not a financial positive, some people prefer solar just to reduce the amount of carbon and other pollutants they are contributing to the atmosphere. As for your case, yeah - maybe the trees make it unprofitable to install solar. Or maybe you could do it, but you'd need more expensive microinverters to help counter the shade. The point is, this tool can tell you. (Well, at least it could, if it were offered in your area.)