Flamestrike
Legend
Let's say the Druid 2 goes for the big guns and turns into a Brown Bear (AC 11, 34 hp). A Minotaur Skeleton (a CR 2 monster) needs 5+ to hit, and deals an average of 17 hp with its greataxe (max 28 hp). It is pretty much guaranteed that the Druid will turn back to humanoid form by the end of round 2 (and would be dead by round 3).
The Druid (with a 14 Con) also has 18 HP underneath the Bear chassis. Assuming the Minotaur hits the Druid (75 percent chance each round) dealing average damage each hit, he needs 3 rounds to drop the druid down to 1 Hit point - at which time the druid uses a bonus action to adopt bear form again gaining an additional 34 hit points (allowing him to soak 2 more attacks).
Without spell casting to heal, the Druid doesn't get dropped till the 6th hit (on average). He survives 8 rounds (factoring in his AC, on average 2 attacks out of 8 will miss). At third level, with Barkskin he survives 12 rounds.
The Chainmail wearing GWF 2 handed Fighter with similar Stats gets hit on a 10+ and has 20HP (plus 7-8 from second wind) goes down after 2 hits (in four rounds on average factoring in his AC of 16 so 50 percent of attacks miss).
The Druid survives for four rounds more than the Fighter. At third level, the Druid survives eight rounds longer.
Damage potential over that same period also heavily favors the Druid. He deals (on average) 8 more points of damage each round he is up over the fighter (barring one round of Action surge).
Every short rest the Fighter can heal (assuming he has HD), and regains one Action surge, and regains one (1d10 plus level in HP) second wind. The Druid also heals (with HD)... and regains 68 hit points thanks to recharging 2 uses of Wild shape. Based off standard encounter pacing of 2-3 short rests per long rest, the Druid (at 2nd level) has a whopping 204 extra Hit points per long rest over what the Fighter can muster.
Moon Druids simply outclass the Fighter at Fighting. They deal more damage, and have a vastly larger pool of HP throughout the day (between long rests). He is also a full caster, and can use Wild shape for scouting and other utility, things that a Fighter simply cannot hope to match.
By the time the Fighter catches up at 6th level (via extra attack, battlemaster maneuvers, access to Full Plate armor and 1 extra feat) the druid is changing into Polar bear form (84 HP per short rest), and gains utility forms like fly (giant vulture or eagle) swim and climb (plus don't forget full spell casting underneath all that combat ability and general utility).
At 10th level the Druid gains an effective 252 extra HP every short rest thanks to Earth Elemental form (and its resistance to non magic weapon attacks). That's a whopping extra 756 HP every 'standard' adventuring day. His damage is still about on par with the Fighter (until the Fighter gets Second attack(2) at 11th level. Remember: the Druid is also now casting 6th level spells underneath all that at this point.
At 'Onion druid' stage, the Druid effectively has a 252 HP buffer that resets every. single. round. And he can now cast his 9th level and lower spells in his elemental form. At this stage it is broken beyond all saving (although few games will get to 20th level).
My point is: you *can* build a frontline tank Druid who will be a frightening biting machine. But that will require an expenditure of resources that could be of use elsewhere.
The Druid above was built with no resource expenditure (via feats, physical ability score improvements to remain combat viable - which he doesnt need - or equipment - magical or mundane - or gold).