Why can harmful bacteria ingested orally colonize the gut so easily but beneficial bacteria can't?

I know this question is a gross oversimplification of the point I'm trying to get across, but it seems to me that there is a weird inconsistency in microbiome health. When it comes to adding beneficial bacteria to the gut, I've heard time and time again that probiotics, which tend to contain the kind of bacteria we want to live in our GI tracts, do not colonize the gut when taken orally. They're transient, apparently, and cannot or will not colonize the gut in any meaningful way. On the other hand, bad bacteria that can make us sick seem to have an almost unnatural ability to latch onto the inside of the GI tract and cause as much havoc as possible, even when ingested in relatively small amounts or even by accident (think eating slightly undercooked food, or drinking contaminated water).

As an analogy, why is it that you can devour probiotic capsules by the thousands and have them make no real difference in the function or composition of your gut microbiome (when consumed at an especially high CFU), but you can drink one palm-full of untreated pond water and be crapping your guts out for weeks or even end up in the hospital or dead? It just seems strange that this dynamic appears to work this way; positive changes are nearly impossible to effectuate along the same pathway no matter the scale of the attempt, while negative changes take just a single slipup or mishap to massively or permanently alter the function of your gut. And if it's the stomach screening out bacteria via stomach acid and Ph, then why are beneficial bacteria so vulnerable to stomach acid while bad bacteria are seemingly unfazed by the same acid?

I know there may be other factors at play (untreated water could contain something other than bad bacteria, like parasites or viruses, while rotting food could contain bacteria that is uniquely damaging or upsetting to the GI tract), but it still just seems odd to me that this dynamic is such a one-way door. Has anyone else thought about this at all?

EDIT: Update second-to-last section with an additional thought.