To me it seems a perfectly natural effect of nearly everyone using it as a website which holds lots of information, and very few people comparatively have any experience with the community side, so people assume that what they see is what Wikipedia is.
Not many people are spending time reading reports on organisation costs breakdowns for Wikipedia, so the only way they'd know is if someone like you actively tells them. I personally also assumed server costs were the vast majority, with legal costs a probable distant second - but your comment has inspired me to actually go and look for a breakdown of their spending, so thanks.
Edit: FY24-25, "infrastructure" was just 49.2% of their budget - from https://meta.wikimedia.org/wiki/Wikimedia_Foundation_Annual_...