Because ultimately the problem with cars is how many of them there are, not what kind of engine they use. If there were only ever, say, 50,000 cars in the entire world we might not even notice the environmental costs. But Google tells me that there are over a billion.
Put another way, a diesel bus carrying 50 people is better for the environment than those 50 people each driving a separate EV car. Not because the bus has less engine emissions, but because it's a more efficient use of materials and energy.
Depends on what they're talking about.
e.g. "Let's plan a murder" vs. "Let's plan dinner"