No clue really, I guess it would be limited to people who can make multiple ranged attacks each turn. It just occurred to me as being one reason you might not want to focus fire. Whether or not it'd still be a good enough reason depends on the penalty imposed for not looking for cover or going prone to avoid said fire.
Part of why suppressive fire works in RL is that one bullet will kill you, and modern weapons put a
lot of bullets in the air. (Yes, you can suppress with non-automatic weapons too, generally requires a semi-auto though.)
And reality check, being outnumbered often does mean you lose. In D&D - "heroic simulator" I think someone called it - the heroes are generally meant to be standing against long odds, with the occasional 4-on-1 boss fight. But one hit doesn't kill them, and they have many ways to overcome being outnumbered (usually magic). So it's hard to imagine a system that would work that combines the mental fear of getting hit and the game-mechanics that "the hit doesn't matter".
And make it not overwhelming to the BEEG when the PCs outnumber it.
Despite all that, I'm gonna just throw out an idea, no idea if it's good...
Suppressive Fire: requires multiple attacks, either from the same source or multiple sources acting in concert. Suppressive Fire fills a 5' square with multiple incoming ranged weapon attacks (and uses 3 times the ammunition of a "normal" attack). Any creature not in Total Cover in that square during its movement (or ends its turn there not in Total Cover) must make a DEX saving throw to avoid being hit. (DC = 8+ lowest attack bonus of the participating creatures). Failure = the target takes 1 hit; for each additional 2 points of failure, the target takes another hit, with a maximum number of hits equal to half the number of "attacks" dedicated to Suppressing that square. Any attacks made from a square that is begin suppressed suffer -2 accuracy, plus Disadvantage if the attacker is hit by the suppressing fire. If any Suppressing source takes a critical hit, they cease participating.
So Legolas (attack bonus +10) with 4 arrows per round Suppresses the doorway the orcs have to come through. He spends 12 arrows, firing as fast as he can. The first orc tries to dash across the opening... DEX Save DC 8+10=18, rolls a 12, and gets hit by 2 (half the 4 "attacks") arrows. The next orc, a raging orc barbarian, braves the doorway (DEX save... 17!), takes an arrow, and stays in place to give his allies cover... new DEX save... 5, and takes two more arrows. Now with some cover against the Suppression, a third orc tries to fire back against Legolas (DEX save 16, +2 for cover from the barbarian), gets hit, but shoots back. He has -2 for being in the Suppression, and disadvantage because he got hit... he misses, and ducks back out of cover.