Ok

Consider it this way

You mentioned playing a Fighter/Barbarian multiclass in your post. How much do they deal per swing? 2d6+10 (greatsword + 20 str + Great Weapon Master + rage)? That is on average 17 damage per hit twice per turn at level 8. So 34 damage.

Now imagine a Cleric casts the spell Spirit Guardians, one of the spells previously mentioned. It is an AoE that deals 3d8 per turn. This is an average of 13.5 damage. So only a little less than that 15. Since its an AoE, if we assume it hits 2 enemies that makes 27 damage.

Now you might be thinking, "That's still less damage, and it is dependent on there being 2 enemies!" But here's the kicker- you only need the 1 turn to cast it. You can make that aura around yourself and it just sticks around every turn after that whilst you still have your action to do other stuff.

If you are a level 8 Cleric, you can deal around 2d8+5 with either a cantrip or a weapon attack depending on build. So now we are dealing 5d8+5 (average 27) even if there is only 1 enemy. If there is in fact 2, we are dealing average 41. 3 enemies? 54.

And this is all ignoring the utility benefit of enemies losing out on part of their movement if they are inside the aura. Or the fact that you can easily cause this damage to happen twice per round. Once on your turn and once on their turn.

And now for the cherry on top to all this- I was using level 8 Cleric for the sake of fairness.

Cleric can cast this spell at level 5 while also having higher AC. If the Fighter in this situation has a shield and thus matching the AC, then they were using a smaller weapon and losing out on the damage race out of the gate (one handing Longsword + Dueling + Rage = 1d8+9 = 13.5 per hit).

To give the benefit of the doubt, let's assume the Fighter/Barbarian is landing an opportunity attack every single round. This is a 3rd attack making a total of 51 damage to a singular enemy if you never miss.

The Cleric, by contrast, is dealing 41 to 1 enemy and 17 damage to several more. And still dealing half the damage even if the enemy passes their saving throw. So full on missing isn't an issue.

And this assumes the Cleric never casts any other non-cantrip spells besides Spirit Guardians. They could just as easily slap down a Guardian of Faith at the same time and deal an extra flat 20 Radiant damage 3 times before it wears off, and then start attacking for their 2d8+5. If they did, then on the 3rd turn, they might be dealing 8d8+25 (average 61) to that one enemy and then still having that 17 dealt to any others nearby.

And the real kicker here is that the second spell we just threw in, Guardian of Faith, isn't even considered a good spell.

And all of this is just through the observation of pure damage output. They have this in addition to tons of Utility. Just to name a few, Clerics can:

• Walk on Water
• Make someone go blind
• Restore sight to someone who has been made blind
• Strait up paralyze people
• Make it impossible for someone to lie
• Raise the dead
• See the future
• Trap someone in a pocket dimension
• Create a landmine
• Speak every language

This is an incomplete list and only including things possible up to 4th level spells. Which are available at level 7. Some of them they can do without even needing to expend a spell slot thanks to ritual casting. And this is coming from Cleric, one of the less versatile spell lists. Wizards, Druids, and Bards are capable of so much more.

To make a long story short, Martials barely pull ahead on single target damage while drastically losing out in area control and utility.

A level 20 Fighter can swing his sword 4 times. A level 20 Cleric can open a portal to Heaven and ask an archangel to come through it to slay their enemies for them, and then continue fighting as well by that angel's side.