I really don't see the point of learning COLL, if you can just do ZBLL. It is way more algs, but probably a lot better. Then again, the recognition for ZBLL is probably really slow.
Every single thing you said is 100% correct. That said, let's consider why every single thing you said is useless for most people.
COLL is 40 algs. And of those 40 algs, basically everyone agrees the S/AS cases are a waste of time, which takes us down to 28 algs. I can learn that in an afternoon, two at most. Figure out which ones don't really help you (spoiler, a good many are actually really nice especially given the increased chance of PLL skip) and forget those, maybe you're left with 15-20 algs. As an intermediate cuber who's just getting into how to learn algs and wants to figure out the next steps to getting faster, this is a fairly easily achievable goal with a pretty favorable effort/reward ratio.
ZBLL is 493 algs. That is a many-months-long learning process for all but the few crazy people who grind through it in a few weeks. Let's not pretend there are very many of those people, and let's not pretend that those people are doing ANYTHING cubing related except drilling algs for that time period. Mid-fast cubers who want to become faster and who have learned how to learn algs learn ZBLL because it's the next step in their journey.
Or we could put it this way:
"I really don't see the point of buying $60 running shoes, if you can just buy $200 running shoes. They are way more money but probably a lot better. Then again, it probably takes a long time to save up that much money."
Yes, $200 shoes should be objectively better than $60 shoes, just like ZBLL should provide objectively more benefits than COLL. However, there's a cost to everything, whether money or memorization/practice or whatever, and many people decide the benefits gained aren't worth the cost to attain them, at least for the present time.
inb4 "ZBLL recog isn't actually slow if you do it right". That's totally fair. Most people only think it's slow, because if it takes months to learn the alg set it's going to take even more months to get the visual recognition down solidly.
inb4 "But you can just learn ZBLL in chunks at a time". What do you think COLL is?
inb4 "I only average 25 seconds and I'm learning full ZBLL before full OLL". Sincere congratulations if you do it. You are again the minority and it's noteworthy but not representative of the larger population.