The game’s systems are built around that assumption. Yes, you can run a game with one encounter a day and it can still be a lot of fun, but a lot of the rules systems will be superfluous (hit dice, for example, would be useless in a one encounter a day campaign), and depending on the party composition, the difficulty could become trivial, or much harder than expected. I hear all the time that 5e is “easy mode D&D” from people who don’t observe the 6-8 encounter day guideline. I never hear that from people who do observe it.
Exactly. Now, let me ask if there's anything wrong with D&D combats being easy?
I'm not saying uninteresting, I'm saying easy. Every and all combat encounters can be fun, different, engaging, and even scary to the players yet they never are in any real danger of a TPK through normal means. All combats are in these massive set pieces with moving terrain, various objectives outside "kill scary monster," and the enemies are roleplaying alongside fighting yet the chance of failure for any given PC is low. It's still possible for them to fail if they aren't engaging or quite unlucky but they know victory often. This can be fun for a wide margin of players. So many players feel awful with frequent failure, why not make failure infrequent?
What if you want to play a resource management game? Embrace it. Resource management games almost always require a time limit put in place. It's the nature of resource management. Change to gritty realism or add random tables. Whatever you do, know that this doesn't mean your game is more balanced than the other, it's just different.
Want to be ultra-hard? Embrace it. Make sure the brutes in your combat can kill an average wizard in one action, create terrain where the enemy is at a severe advantage, introduce insta-kills into your campaign.
Reinforce the themes you're set to establish in your campaign. Don't try making a resource management game but avoid forcing the players to manage their resources carefully. Don't promise an easy game and panic when players are whistling their way through encounters. Make sure you know the game you run.
The game was not created to be systematically balanced, the game was created to be fun through a cooperative experience with your DM and fellow players; be it to tell a story, face challenging quests, or experience and explore a world that your character is invested in.
In short, you don’t need to conform to that guideline, but if you don’t, your gameplay experience will be very different than the one the game is written to accommodate.
That's good. Embrace the difference. In easy campaigns, the spellcaster might not even need charm spells since everyone's so friendly that even the low charisma barbarian can autosucceed in their speech ability. In hard campaigns, the spellcasters have to think if they can afford using a spell slot for a high-level spell that might fail on a OOC situation or use a low-level spell but not be as convenient or conserve both so they have them for an upcoming fight. In medium campaigns, it might be a mix.
But diversity between tables is good.