Why play melee champions when ranged champions have better laning phases and better late games?

Because every team needs a tank, someone to soak damage so the carries don't have to. And there are no ranged tanks (aside from and but Gnar is half melee anyway and Graves is just bullshit). So naturally, someone has to play a melee champ. Even if it's something like Riven or Fiora, who aren't actual tanks, those few extra seconds they become meatshields might give the mid laner/ADC enough time to do their job.

why play a juggernaut when is literally 100% a ranged juggernaut?

Because melees are what allow ranged folks from having the "better" late games

omg adc is so useless! omg range is too strong!

ok board.

to maintain a sense of dignity

Melee isn't versatile the same way projectile based champs are (unless tailored to be a certain way like Vladimir for instance).

Ranged chmps are slower and are generally vulnerable to zoning and collapse. They have lower health and scaling defensive stats. It's usually good to have someone who can stay in melee for ranged champs.

But nothing is set in stone. META changes slightly with team comps.

melees are just meant to be bullet sponges for our superior ranged overlords, don't you know?

I think I see, now. Gnar is ranged champion who falls into a different category than marksman like how even though Ezreal is tagged as a marksman, he is almost purely mage.

Gnar is special and unique from other champions in the way that he... turns into mega gnarly, gains access to 2 stuns, Shuriman Librarian sized health bonuses, and has no resources tied to abilities making him a good sustain lane. What sucks about Gnar that Garen has over him is Damage Red., an execution, and the ability to cleanse all slows effecting him on a short cool down, which just makes him great at tAking down high priority targets (which are usualy ranged) with little damage sustained. Not only are his cooldowns very low (a trend with melee champions are there relatively lower costs (In the case of Garen and Gnar, cooldown is a resource and fits this rule of thumb), but Garen is also given a bit of a range boost on his cooldowns execute.

Tl;Dr it's about balance. They still fixing it.