In a fascist dictatorship, they have a lot more in common than opposition.
But if the dictatorship is a communist one they have more in common with the nazis! Or if your country is invaded by Russia you might find yourself fighting side by side with the Azov battalion.
There are libertarians who genuinely care about free speech and might make useful allies on those issues.
Just because someone is the enemy of your enemy, or an occasionally useful ally, doesn’t mean you want to unify with them.
The idea that all “leftists” should just work together is stupid.
Leninism, Anarcho-primitivism and Social democracy (for example) are not different approaches to “leftism” that ultimately want the same things; they are completely separate ideologies that naturally come into conflict. The people who follow them disagree with each other because they want and value completely different things. If they were to put aside their differences there would be nothing left.
That doesn’t mean arguing on the internet about ideology is meaningful, or that there can’t be common goals or enemies, just that you should give up the idea that all “leftists” are somehow natural allies, because it doesn’t make any sense.
I was considering putting scare quotes around “communism”, but refrained in order to avoid an argument about what is and isn’t really communism. Yet here we are. So much for left unity! ;D