[ETL]

Some have tried to say the teleporter was the show’s vision of the future. In fact, they couldn’t afford the special effects of landing a ship so this was a cheap alternative...

Same thing I imagine for all the M3 ("earth-like") planets they encountered. Rarely was there the need to wear life support equipment.

[ETL]

Spock: Jim, it appears to be yet another M3 earth-like planet. Atmosphere and other conditions identical to earth.

Kirk: Of course it’s an M3, you vulcan dope. You know they can’t afford the more elaborate sets and special effects.