I think you may be making an assumption that folks who carry a lot (subjective) of ammo are looking for a reason to “spray and pray”. Which is generally not the case.
Let’s look for example at the Glock 19 which is easily one of the most popular carry firearms. It’s popular because for most people it is a full grip, offers a decent barrel length, and is still very concealable for a lot of people. It carries 15 + 1 rounds standard. Carrying a single spare magazine (which is good practice) brings you to 31 rounds. Carrying a second spare, which for a lot of people is no extra trouble, brings you easily to 46 rounds. If you choose to carry the 17 rounds mags as spares instead of the 15 round mags, it costs you exactly nothing in comparison to carrying the 15rounders and brings your total carried to 33 (1 spare) & 50 (2 spares) rounds. Many people would say “that’s a lot of rounds!”, but really it’s just 1 or 2 spare mags.
So it’s not that folks are necessarily thinking they will be fighting insurgents at the local 7-11, it’s just that it costs them next to nothing to carry a spare mag or two and if you happen to need it you have it. If you don’t need it (as happens 99.9% of the time) you haven’t lost anything.
As for possibly scenarios where you may need a lot of rounds…
You are facing a determined attacker. That may be because they are doped up on drugs, but it doesn’t have to be. This normal size guy who was not high on bath salts took 11 rounds to stop. The Dayton Ohio mass shooter took thirty rounds from a handful of cops before he stopped (I do believe he was doped up). The police shot (I think) 48 rounds, so the hit rate was very high.
You may be facing multiple attackers. There was a guy last year in Houston who faced down five armed home invaders with an AK47. I don’t recall seeing an accurate count, but news stories said “dozens of rounds fired” and “piles of brass in the front yard”. The invaders were shooting back at him as they drove away, so they were still a lethal threat and return fire was warranted and no charges were filed.
Heaven forbid, if you have to face a mass shooter… For sure, they will have more ammo than you. They’ll be prepared and strapped with ammo in a backpack, tac vest, fanny pack, etc. If all you’ve got is a 5 shot revolver and no reloads you’ll have to make do, but you will certainly wish you had more ammo.
Now, I’m not saying these are likely scenarios, but they are certainly possible. And if it doesn’t cost you anything to carry “a lot” of rounds, then where is the harm in bringing them along? This is where everyone has to come up with their own risk profile and how they feel comfortable managing that risk and what they will/wont carry to address that risk. Some people are OK carrying 50+ rounds, others are OK carrying 5rounds. To each their own.
I worry some folks may get the idea that many rounds fired makes up for a lack of accuracy
I don’t think anyone here falls into that category. I don’t think I’ve ever met anyone (even those who claim to carry “a lot” of rounds) who falls into that category.
the military itself fell into that trap years ago which is why the troops carry poodle shooters rather than battle rifles-BUT the military uses massed fire to pin down the enemy so artillery and air strikes can be deployed
This, um… seems inaccurate? Maybe there’s a crayon eater here who can clarify better than I can… Mil learned midway through the last century that limited accurate fire is better than “massed fire”. Once GIs started dumping mags into the jungles of Vietnam that 3rnd burst switch was quick to replace full auto. We do have a doctrine of suppressive fire when needed, but usually from dedicated platforms like the SAW. And if you are close enough to pin down the enemy with small arms fire, you are probably danger-close for artillery and air strikes.