It's not the only reason their traffic is declining, but it seems like a big one.
[1] https://en.wikipedia.org/wiki/Wikipedia:List_of_citogenesis_...
Google shifted views that used to go to Wikipedia first to their in-house knowledge graph (high percentages of which are just Wikipedia content), then to the AI produced snippets.
All to say, yes...Wikipedia's generosity with outbound links is part of the popularity. But they still get hit by this "engagement" mentality from their traffic sources.
In terms of a single search, I don't think Google really benefits from preventing a click-through - the journey is over once the user has their information. If anything, making them click through to an ad-infested page would probably get a few fractions of a cent extra given how deeply Google is embedded in the ads ecosystem.
But giving the user the result faster means they're more likely to come back when they need the next piece of information, and give them more time to search for the next information. That benefits Google, but only because it benefits the user.
People come to your site because it is useful. They are perfectly capable of leaving by themselves. They don't need a link to do so. Having links to relavent information that attracts readers back is well worth the cost of people following links out of your site.