Even if 95% of the spam gets actively reported and dealt with, that still leaves a ton of nonsense on the platform, getting fed into the LLM. And spam has only gotten worse over the years, as the barrier to entry has lowered and lowered.
One of the most interesting things that I've noticed is these advertisements will be triggered if you follow accounts that are positioned as influencers. I followed one out of curiosity and received a DM from that account advertising some cryptocurrency service.
It's a good way to filter out and block accounts that have almost certainly not grown organically.
You know people lie, right? Especially when the lie casts them in a better light and/or makes them more money.