D&D Combat is often very dull. Either in a cave, dungeon or out in the desert, you are just slogging it out, often repeating the same spell or weapon attacks repeatedly until you deplete the monster(s) HP. The more complex monsters and monster team-ups can make this better, but players are still left with the same choices over and over again.
Terrain! The most cinematic combat in movies, TV, books, and other media always involves the hero struggling against incredible odds, to survive and carry the day. Sometimes the heroes overcome these long odds with skill or overwhelming power, but more often than not in fantasy, it is done with cunning. A well-timed drop of a chandelier onto the Count's head, toppling over a pillar onto a dragon in some old ruins, pushing the evil goblin into a pit of lava while fighting in a caldera of a volcano, or blasting a horde clumped together as they chase you through a narrow tunnel.
Yes, and no. There are a few places where terrain features are just not likely to be, such as a plain old vanilla dungeon hallway OR an open stretch of the desert (aka barren/sterile terrain), but those are the exceptions.
A forest glade will have thorny brambles (survival to get through safely), dry brush (perfect for being set ablaze), slippery mud patches, teetering rotting trees (athletics to push over), rivers/streams (not everyone can swim), a bee hive, etc (See Forst Terrain Random Table). A rocky desert will have spires of rocks/towering cliffs (keen to be shattered/pushed over), vipers burrows (better to poison you with), groves of cacti (spike growth without a spell slot), etc. I could go on.
I agree planning combat is hard, and the CR system is a helpful guide but each party has different strengths and weaknesses which you have to adapt to encounters to account for. Terrain can help to balance encounters by providing tactical options to even the playing field for either side. Say your party is wiping the floor with your solo monster... have him knock down a tree to separate the party giving him a fighting chance. Let's reverse that, the hoard of monsters has quickly dropped a few party members, and the action economy advantage is in a downwards spiral, well have the ranger roll a survival check to notice a bee hive dangling from a tree amidst the hoard.
Too true... but there is no such thing as wasted prep (you can always use it later) OR skip the prep and roll-on or pick from a random table. However, if your players are ignoring the terrain make sure to remind them it's there by having your monsters use them. Throw a few "low risk" encounters at them at the beginning of a campaign with "easy" monsters which use terrain against them, and your players quickly start taking advantage of them too. BTW the same goes for tactics... if you want your players to be tactical, have some easy monsters use tactics against them.
After a while of being reminded (by combat or with your descriptions of the area), your players will become suitably trained so that they will start to seek these things out. Are any of the trees around here rotten? Do some of the rocks on the mountainside look unstable? Encourage this, as it allows you to share the creative load, by allowing the players to find the things they are seeking as long as they are reasonable, and if they are not reasonable use it as inspiration to give them a reasonable alternative.
That's the rant. If you are a player start looking for that advantageous terrain, if you are a dungeon master "remind" your players that it's there, and if you are neither grab some dice, some friends and play some D&D.