Street trees aren't car-supremacist enough.

Let me explain what I mean by that: when a driver fucks up and his car careens off the street and hits a tree, the tree stops the car very abruptly. That's great for, say, an innocent pedestrian who was saved by hiding behind the tree, but can apply rather serious consequences to the negligent driver. Car-brained traffic engineers see it as their mission to protect drivers from any and all consequences, so they insist on ripping out all the trees to create a gigantic "clear zone" so that the car is free to careen wherever it wants without hitting anything solid. Squishy things within the clear zone, such as pedestrians, don't enter into consideration.

In other words, one important "advantage" of these "liquid trees" over real trees is that they can be mounted on breakaway stands, so that they yield (and therefore provide no protection to any hapless bastard who might've been sitting on the bench at the time) when a car hits them.

Source: I'm a former traffic engineer. But don't take it from me; watch this confession from a much more experienced and credible engineer explaining it in even more stark terms.