> We also jail the most outspoken jihadis,
I believe the question here is influence. There is no jihadist party in Europe, but there are plenty of parties (some in governments, some even with almost full control of their countries) that are nazi in everything BUT name.
> The far right has fairly high preferences in the last decade or so, so in practice, you have a lot of far right politicians in parliaments and sometimes governments.
Outrage drives engagement and makes media more profitable. This is why things like Fox News and the Murdochs exist. Now they have discovered they can influence elections as well. We can blame Russia for giving a helping hand to far-right authoritarians aiming to destabilize the EU (and the US, let's not forget that).
> Jailing people don't make them disappear,
No, but it sends a message.
> Ideologies are somewhat contagious. Unless you isolate the carriers socially, which is hard to impossible, they tend to infect new people.
Like any disease, it's hard to fight and, like any contagious and deadly disease, it's worth fighting.