Answer the first question first.
Because a Druid can swing weapons.
I'm going to be honest, I don't really see anything a Ranger can do that a Druid could not, outside of having better fighting abilities, and well, in 5e design, that should really make Ranger a Druid subclass more devoted to combat (like the 3.5 Druidic Avenger found in Unearthed Arcana). Put another way:
Fighter with a focus on wilderness skills and stealth.
Rogue with a focus on wilderness skills.
Druid with a focus on combat and stealth.
Oath of the Ancients Paladin with wilderness skills and stealth (maybe allowed to smite with a bow?).
Any three of these could be a viable substitute for the Ranger class. Because here's the thing; even if you buff the exploration tier to make wilderness skills more important, you're still going to need other kinds of characters to interact with it. So you end up with the Rogue, who sure, they can use thieves' tools, find traps, and sneak about really well, but so can other characters.
So even the "Wilderness Warrior" some people want wouldn't be the unrivaled master of exploration and survival- they really couldn't be, because that design would make having a Ranger necessary for play, which isn't the way 5e is (supposedly) built.
I've played Rangers, I don't mind Rangers, I even like the archetype- but there's nothing really unique about them, other than being, in effect, a multiclass character who isn't actually multiclassed.*
(EDIT: you know, like the Bard. Or to paraphrase Order of the Stick: "I'm a multiclassed Fighter/Rogue/Enchanter!" Bard: "Don't you find that overly complicated?" "I didn't until this very moment, no.")
I feel the same about the Paladin, to be honest. Sure, the Paladin has more identity and some really nice abilities, but there's no reason, in my mind, that you couldn't make a Divine 1/3 caster Fighter subclass akin to Eldritch Knight, or a Cleric subclass that actually gets Extra Attack (why War doesn't when Bards and even Wizards can have Extra Attack still boggles my mind) and call it a Paladin/Champion/Holy Warrior/Templar/what have you- but obviously that wouldn't fly because players
want a Paladin class. That's the only reason we still have a Ranger, as near as I can tell. Not having a Ranger class doesn't "feel like" D&D, even though we really don't need it.