Note I write this from the perspective of someone who had 8k followers on Twitter and hung out with lots of other "influencers" in my own little niche community (webdev). But this is just my perspective.
1. FOMO. You really do miss out on "what everybody's talking about." Even if what you're missing is just some hot drama, often a result of Twitter's UI decisions themselves goading people into fights/harassment/outrage/dogpiles/etc. It still feels bad to "miss" it.