Well, there are a variety of good reasons why real-name policies aren't always the best idea. The "authentic real self" approach works okay if you've got the social and political capital to weather any negative consequences of your statements being directly associated with your real identity, but that's not true for many/most(?) participants in our democracy. For example:
* a queer youth in a deeply conservative area takes their life in their hands by being authentic w.r.t their sexuality
* a conservative employee of a liberal company risks their livelihood by expressing beliefs espoused by a mainstream political party
* a feminist activist is stalked by creeps who disagree with their legitimate speech and have their dox at hand thanks to the real-name policy
You don't need to reach very far to find plenty of specific instances of the above issues. Furthermore, that's limiting the conversation to the American political arena. The issue is immeasurably more severe in areas of the world where political violence is a fact of daily life. One commonality is that the chilling effect is strongest among marginalized groups - real-name policies wind up as tools for majoritarian oppression, as the opinions that are most safe to voice are the ones that are the least controversial. This is just a small sample of topics in this fairly well-researched space; check out[1][2] for a lot more depth.
I guess what I'm getting at is that a real-name policy would work great if negative consequences only accrued to genuine bad-faith actors like bots and propagandists. Sadly, that's not the case - completely civil, legitimate speech is chilled by the threat of physical violence, social crucifixion and financial disaster enabled by gratuitous real-name policies.
On top of that, it's not like real-name policies are some kind of panacea for the problems you mention. Facebook aggressively enforces theirs, and they were hardly immune to the problems Twitter has dealt with in/since the 2016 election. If your concern is more around people saying stupid shit "around movies and political events" - again, take a look at Facebook. Your great-uncle Larry doesn't care that everyone knows he thinks Obama was born in Kenya.
This is why I'm more keen on alternative solutions, like the identity-vouching providers discussed up-thread. Instead of a real name acting as a proxy for legitimate interest in participating in a given conversation, use a trusted third party to establish that a given account really is, say, a voting American citizen with service-specific hash s (to avoid sockpuppeting) and not some guy in a basement in SPT. That way, you get some of the hypothetical benefits of a real-name policy w.r.t cutting down on abuse while maintaining the freedom of expression afforded by pseudonyms.
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[1] https://www.eff.org/deeplinks/2014/09/facebooks-real-name-po...
[2] https://www.eff.org/deeplinks/2011/07/case-pseudonyms