In our own solar system Mercury, Venus and Mars are a single type of habitat or environment. They'll kill you without a spacesuit. But people seem to enjoy pplanets that are entirely modeled after a relatively small area of Earth. this has some justification.
It's a good shorthand way of writing up a planet for a referee under the gun. Referees are always short of time and energy and any established shortcuts like this are usually overlooked. If you're a writer and try it you're called a hack. Or worse.
In fairness and entire campaign might take place in a relatively small area of a single planet. If a group of characters had a series of adventures on an Earth like planet but in a region resembling the Sahara in size and conditions they might walk away thinking it was all desert; and for their purposes it was!
Sadly we do not have a lot of so called garden planets in the Solar System. Just the one (and we may be voiding the warranty on this one soon). Could a planet be a single biome? There are a couple of factors that work against this.
Planets are big! This might seem obvious but most of the other reasons planets may vary hinge on this one fact.
1) Most planets are going to have seasons due to orbit and axial tilt.resulting in different temperature ranges for different latitudes. This will affect life in the different areas.
2) Water and other vital resources may not be evenly distributed. Okay on the bright side oxygen and air pressure are pretty constant across the globe though they vary by height. Wether patterns (that depend on sunlight in a large part) and location will deny water to some locations and dump too much in others.
3) Land forms and water ways may isolate animals and plants creating different eco-systems in different areas.
Some planets might have a more even distribution of water as well. Law of averages and all that.