I think perhaps the main reason that ranged combat doesn't dominate in most 5e games is simply because neither the players nor the DM's want it too. Most players (I think - I could be wrong) don't approach it solely as a tactical number crunching game, and most DM's don't run the bad guys as squads of special forces rangers and snipers.
A lot of players just don't think about it that way. They think "I want to play the badass barbarian who wades into melee" or "I'm the wizard who confounds the enemy with his magic" (Ok, maybe it's only my wizards who actually use the word "confound").
They build a melee fighter, barbarian, ranger, paladin, or rogue because they want to be in melee, not because they've run the numbers and it's tactically more advantageous. It's simply more fun for them. If they play an archer they do it because they feel like playing a ranged character
<snip>
It's just not going to matter for most groups, because the guy who likes playing barbarians is still going to be a barbarian, no matter what. That's the reason he plays the game. The guy who likes playing archers was going to do that anyway.
This is one of the things that makes Mounted Combatant an attractive feat for melee-oriented characters. Melee is normally at a relative disadvantage in open terrain, but Mounted Combatant and a good warhorse together provide mobility, better weapons (lance), and advantage on many attacks whenever you're in open terrain. You can of course do this without any feats at all, but Mounted Combatant eases the logistical aspect (don't need to keep buying fresh warhorses) and provides advantage on many attacks.
These two posts remind me of what is - for me - the great paradox of D&D: it is quite a way down the crunchy, non-transprarent end of the RPG spectrum, yet has more casual or non-crunchy players playing it than all other RPGers put together!
To elaborate: the person who just wants to play a barbarian, an archer, or whatever, would be just as well-served by a loose descriptor system where there is a single character-sheet entry - say "Greataxe-wielding barbarian" with a number next to it. The intricate D&D subsytems of attack number, damage number etc are irrelevant.
But for better or worse D&D doesn't work like this; which is why such a weight falls on the shoulders of the designers to try to ensure that when that sort of player engages the mass of subsystems, the results are broadly what the player was expecting.
The non-transparency is another conributor to results here: the game doesn't just come out and say "If you want to play a melee warrior on the open plains, make sure you take Mounted Combat!" It leaves it all for the players to work out - which makes the risks of outcomes that don't match expectations even greater, and hence amps up even more the pressure on the design team.
4e tried to defuse some of that pressure by increasing the transparency and hence shifting some of the work back onto players (along the lines of "If you do X, Y and Z your PC will work fine under paramaters A, B and C; you can build something else if you want to, and it might work out fine or even better, but if you're going to take that pathway the onus is on you").
But 5e doesn't. The only onus-shifting going on is from the design team onto the GM; but that's potentially quite unreliable for all the well-known reasons, including that asking the GM to make sure that a given player gets what s/he wants out of his/her PC can easily push up against all those intricate sub-systems.
Frankly, I think it shows the calibre of the WotC design team that they've been able to publish a mostly functional game, given these ludicrously extreme constraints under which they're designing it!
A system cannot mandate, nor enforce, parity. It is incumbent on the players around the table to do so.
I think a system can go a fair way, actually. Eg you can tone down all the interacting, unexpected-feedback-generating subsystems (an early example is Runequest, or Traveller if you're happy to jump genres; Dungeon World and HeroWars/Quest are more contemporary examples).
Or, you can build a system where weaker PCs generate GM resources able to be used against stronger PCs (Marvel Heroic RP aims for this: players whose PCs have smaller dice generate more Doom Pool dice for the GM, who can then use those dice to confront the upper-end PCs).
Or, you can set up your instructions to GM and your player resource suites so that differences of mechanical effectiveness between PCs become secondary (Burning Wheel does something like this - in BW,
engaging the in-game situation is far more important than
succeeding, both in terms of shaping the ongoing fiction and PC development).
Or there is the 4e approach: like 5e, damage capabilities are carefully calibrated (in 4e on a single-encounter basis rather than a 6-8 encounter basis); but then clear non-DPR functions are expressly called out and allocated in pretty big, clear chunks, so a player who builds a non-DPR character is clear on what s/he is getting by way of "compensation" for lack of damage-dealing capability. 5e, by dropping this transparency, has made it easier for disparity among PCs to emerge accidentally/organically.
