And I should read more, and post less. I literally just wrote the exact same thing (except no where near as concicsely) before reading the comments. I'll snip and put it here, essentially as a rephrasing of the exact same concept:

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It's because silence is perceived as agreement.

When some extreme position (perhaps on a social issue) gets espoused by somebody with left leanings, there tends to be little to no pushback except from right leaning individuals. This gives one the perception that there is a mixture of apathy to agreement with said views. Similarly, if one does push back against said views you're immediately bucketed as a right leaning individual, or "worse." So it all becomes a bit of a self fulfilling prophecy.

This is why any sort of censorship, or self censorship, really just ruins (little d) democratic discussion and debate. You have no idea what people "really" think, so you're kind of left to just imagine it. And we mostly get it completely wrong. See the Perception Gap: https://perceptiongap.us/