Right. The issue isn't that the Fighter can't climb trees with an Athletics check. The issue is that a spellcaster can cast Mage Hand without even rolling a dice.
Let's be clear:
Save DC - Acid Splash, Poison Spray
Spell Attack - Chill Touch, Fire Bolt, Ray of Frost, Shocking Grasp
These are, for all intents and purposes, attack spells. This is what a Wizard can do one of these things, every turn, in combat. These things also scale. Just like Extra Attack does. These spells are also Cantrips, and cost zero resources, just like an Attack action. You can do them infinitely. We'll also remember that a Dragon Sorcerer at Level 6 will gain their casting stat to damage as well. Then of course no-one should ever forget that it's a Warlock, that is commonly understood to be the baseline for damage.
When a Wizard hits 5th Level, all their Cantrips are boosted, of course. But, additionally, let's say that twice per day, as a resource, they can cast Fireball in combat. Secondly Fireball sets everything on fire (DMs forget this...The book that was on the table, that the party was sent to get? It's burned to a crisp now because you cast Fireball in the boss' room. Quest failed.), which has several uses. I'm ignoring 1st Level spells per day that they can also do. And I'm ignoring the 2nd Level spells per day that they can also do.
At 5th Level, in combat the Fighter gets to umm...Yeah...Twice per day. Not equal.
Spellcasters are fantastic in combat. Anyone who says otherwise, is simply not reading the book.
Now, the major balance issue, is that Spellcasters aren't just good in combat. They can do basically anything.
No resource abilities, at-will (Wizard):
Dancing Lights, Friends, Light, Mage Hand, Mending, Message, Minor Illusion.
I'm not even going to discuss leveled spells. But there you go. Those are Cantrips that every single Wizard has access to, and can choose from, in addition to choosing whatweaponspell attacks they'll have. ...Also Wizards can make weapon attacks, too, but y'know. Let's stay focussed. They even get more Cantrips as they go along. Again, this is what a Wizard can do outside of combat, without spending any resources and largely not even rolling any dice for a chance of failure.
A Fighter, chooses one Fighting style...And it wasn't until Tasha's that they could even change it (without DM fiat), and even then they can only change it every 4th Level.
The magic system needs to be neutered. First thing that needs to done is to hard limit each individual character to particular schools of magic, so that spellcasters can do anything, but they can't do everything.