I think I know the problem with Garen.

He feels bad in longer fights ( something juggernauts thrive in) because of his abilities and cooldowns.

Now I'm not saying increase his damage, but take a look at how other Juggernauts play in sustained fights.

Can constantly use Q, pull enemies in, and can ult a lot if he gets resets. Get that passive going and feel amazing in fights.

Constantly spams Q, has an active sunfire, a nice auto reset, and his ult makes him feel awesome in sustained battles.

Can use W a lot to get tentacles slamming, can Q tentacle, can pull spirits, and ult makes it feel a lot more fun and epic with constantly having something to do.

Though he has most of his damage in Q, constantly being able to use it feels nice in fights, throws down E for armor shred, wither enemies, and ult to get huge and tanky.

Ignoring how shitty he is he can constantly spam E, use W to sustain, Q feels awesome with each hit, and his R feels amazing, especially when he gets a soul.

Now look at Garen.

Qs every so often in fights, uses E and W, if the enemy isnt in kill range he awkwardly stands around and auto attacks while waiting on his E to come back up.

He has too much downtime between his combos and sustain in fight that it's better to build him full AD just to kill the enemy in 1 combo or they will kill you easily.