Now I finally understand why, in Star Trek TOS, they only visited planets with "lifeforms less advanced than humans" (or with no human-like lifeforms). Bugger the Prime Directive, and the mission to "explore strange new worlds". Doing otherwise was just too dangerous!
Can you imagine the perils of trying to keep a 300m-long starship in orbit, without hitting all the bits of rubbish? Sulu would have been doing slaloms for half of every episode. (Although they could probably spring-clean a planet of orbital debris within a few hours, too, just vacuum it all up with a tractor beam).
As Spock so eloquently put it in ST4: "Judging by the pollution content of the atmosphere, I believe we have arrived at the latter half of the 20th Century."